Although not the oldest son of Kavadh I, the terms of his father’s will and the support of aristocrats and Zoroastrian clerics led to him being crowned in 531 c.e.
In the second year of his reign Khosrow agreed to an "eternal peace" with the Byzantine emperor Justinian I. By the terms of the agreement the Byzantine government subsidized the defense of the Caucasus Pass, which had been used by Berber tribes to attack Persia and Byzantium.
Byzantine support for rebellions in Armenia and Georgia against Persians convinced Khosrow to break his peace agreement and begin a second war with the Byzantine Empire in 540.
After establishing another peace, Khosrow successfully fought the Romans in Lazicia on the Black Sea and in Mesopotamia until 562, when a 50-year peace was established. Using the peace between Persia and Rome, and with a coalition of Turks, Khosrow defeated the Hephathalites, a permanent threat for Persia.
Khosrow made many reforms while king and continued Kavadh’s attempts to reform the taxation system by abandoning the annual taxation calendar, introducing instead a constant system of taxation that was based on a survey of property and annual income. Changing the taxation policy gave Khosrow the ability to make longterm plans for the country.
Because of his attempts to establish social justice, Khosrow became famous for his just rule. Khosrow I organized a permanent army whose discipline was superior to that of the Romans. He was also the first Sassanid king to pay a salary to soldiers and provide weapons.
In order to minimize the risk from plots against him Khosrow divided the empire into four regions, each of which was ruled by a military leader. During this time Ctesiphon, his capital, became a metropolis, and he developed his famous palace, Taq-e Kasra.
In addition to founding new towns, Khosrow constructed buildings, canals, and strong fortifications in frontier towns to protect his empire. Although Khosrow was an orthodox Zoroastrian, he was tolerant of other religious beliefs.
Because of Khosrow’s interest in philosophy, seven Greek Neoplatonic philosophers immigrated to Persia after the Academia of Athens was closed in 529 by Justinian I. With his support, many books from India, Greece, and Syria were translated into Pahlavi. One of these books, Kalileh and Dimneh, remains one of the most famous works of Persian literature.
In his last years Khosrow extended the boundaries of his territory to Yemen. The Romans stimulated the Turks to attack the eastern boundaries of the Persian Empire, and then attacked Mesopotamia, part of Persia. Despite his old age Khosrow personally led the Persian army and defeated the Romans.
He went on to conquer Armenia, Syria, Cappadocia, and the fortress of Dara on the Euphrates, before forcing the Byzantine emperor Tiberius II to sign another peace treaty. In 579, while negotiating the peace contract, Khosrow died, and his son, Hormozid IV, succeeded him.