After Diocletian proclaimed himself emperor, he changed the head of state system into an imperial system. The monarch wears a crown, precious silk robes and gold and silver jewelry, and his subjects need to bow down to the monarch. In addition, the monarch is worshipped as a god. For example, Diocletian called himself the incarnation of Jupiter. In addition, Diocletian was ruled by four emperors. There were two emperors in the east and west, one was the chief named Augustus and the other was the deputy named Caesar. The supreme power belonged to Diocletian. It is also stipulated that the adopted son and son-in-law whose deputy is the chief position should give way to the deputy if they have been in office for 20 years. In this way, Augustus Diocletian of the East was stationed in Nicomedes, ruling Thrace, the East, Asia, Egypt and Bentu, and Caesar Galerius was stationed in Thessaloniki, ruling Macedonia and Mercia. West Augustus Maxim was stationed in Milan and ruled Italy, Laetian, Norik, Illyria and Africa, while Caesar Constantine I was stationed in Trier and ruled Britain, Gaul and Iberian Peninsula. Diocletian also reduced the size of the Federation, divided more than 100 Federation, and established administrative districts, with more than a dozen federations as one administrative district, and local governors did not hold military posts. One of the most notable events during his reign was the invasion of Ilanchar in 297 and the occupation of Taixi Peak in 1998.

Diocletian reformed the army and divided it into border guards and patrol troops. Border guards are used to resist foreign invasion, and patrol troops are used to suppress people's uprisings and rebels. He also reduced the size of the legion and increased the number of legions sent. Because Diocletian divided the empire into four parts, each emperor had a large number of soldiers, which greatly increased the economic burden of the empire. Therefore, Diocletian introduced a new tax system. In terms of poll tax, it is stipulated that adult men have one head and women have half a head. Land tax is levied according to the type and area of grain fields or orchards, and urban people are taxed according to various industries. In addition, officials, retired veterans and slaves are exempt from taxes. In order to ensure the tax source, Diocletian promulgated laws to prohibit farmers from moving freely, and craftsmen and businessmen changed careers, even stipulating that they must inherit their father's business. Diocletian also changed the monetary system and adjusted prices, but none of them succeeded. In 300 AD, the population under Roman rule was only 40 million.

Diocletian abdicated in 305 after 20 years in office, and so did Maxim. In the west, Constantine I, the enthroned monarch, died in England just over a year after his reign, and his son Constantine I succeeded him. When Constantine I ascended the throne, he only occupied Britain and Gaul, and the rest were under the control of Maximus' son Maximus. 3 12 years, defeated Maxitius and unified the west. In the East, after the death of Galerius in 3 1 1, Signius, who ascended the throne, defeated Maximinus Deya, who occupied Egypt and some Asian territories, and unified the East. In 3 13, Constantine I and Signius issued the Milan decree, declaring Christianity legal. 3 14 there was a conflict between them and there was no draw. In 323, the two men fought again, Leonius was defeated, and Constantine became the sole ruler of the empire.

Four Emperors Rule (English: Tetrarchy) is a governance model founded by Roman Emperor Diocletian. Diocletian was an ambitious heroic emperor. Because he saw the drawbacks of the hereditary system of the Roman Empire in the past two or three hundred years, the whole country was weak. So he carried out a large-scale earth-shaking reform of the political system of the whole country, that is, he abolished hereditary monarchy and autocratic monarchy, and innovated a four-emperor governance model: the emperor appointed two supreme leaders (or two emperors who shared the emperor) as Augustus, and two secondary leaders (deputy emperors) as Caesar, that is, the whole * * * had four leaders to divide East and West Rome. Augustus' term of office was limited, and he abdicated automatically after the expiration, and was succeeded by the commander-in-chief. In this way, we can avoid the bad habits of someone who has been in office for a long time, such as corruption and greed, and the dilemma that it is difficult for the emperor to control the local government.