Having completed the League One season with Chinese side Beijing Renhe, the full-back has hung up his boots, aged 39.
Sun made 130 appearances for the Blues where he spent six years, from 2002 to 2008, scoring for the Club on four occasions.
He was the first Chinese player to score in the Premier League; heading home in a 2-0 victory over Birmingham City in October 2002.
The defender was a fan favourite, helping City to win the First Division title in 2002.
In September 2015, it was announced he would take up the post of Club Ambassador in China, promoting the work City are undertaking within the local community and with their local partners.
He was inducted into the National Football Museum's Hall of Fame last October.
We wish the best of luck to him for his future ventures.