He was the grandson of King Wonseong, his father Kim In-gyeom having died before he could take the throne.
On his deathbed, King Soseong, he called his younger brothers, including Eonseung, and told them that he would hand over the throne to his son as his grandfather had wished, and that they should respect the succession.
However, this was ignored as Aejang was usurped, which brought on a renewal of the succession strife which typified later Unified Silla.
He was appointed to the Si-jung in October 791, but retired due to illness in August the following year.
When Uncle Uiyoung died early in 794, he was proclaimed Crown Prince in the year 795.