What is the background music of Su Qiqi Su Liuliu? Once in a thousand years. According to qq music query, Su He is the two heroines in the TV series The Longest Day in Chang 'an, and the background music is Once in a Thousand Years. Once in a Millennium is a touching ancient song, written by Chen Zhenchuan and composed by Hu Weihong. This background music properly interprets Su He's temperament and character, complements the plot and is deeply loved by the audience.