Sun was born in Shou County, Anhui province in 1912 to educated and middle class parents. Her original name was Sun Yunjun (Chinese: 孙韵君). Her mother ran a girls' school and her father was a secretary to Sun Chuanfang and he was later a professor. Sun had originally intended to study literature after completed her schooling at Anqing Girls School but she was unsuccessful at entering National Central University. She did visit the university to study painting where she took lessons from Xu Beihong. The art professor with a partner and family regarded Sun as a "painter of genius". He admired her work and invited her to pose for paintings. Xu shared the news of his new love with his partner Jiang Biwei in 1930 who decided to not make this an issue (Jiang was betrothed to someone else before she eloped with Xu). Xu was now a Professor of Art at National Central University.
Sun became a full-time student with Xu Beihong in 1931 and their established affair was well known. There were complaints about the extra attention that she got from her teacher and of how they both ignored the rules concerning male visitors to the university's female dormitory. Sun was obliged to stay with her aunt in Anqing and not live at the university. However, she continued to attend lectures whilst Xu Beihong travelled abroad in January 1933. Xu and his wife Jiang Biwei toured eight countries with Xu's exhibition Chinese Contemporary Paintings. After Xu returned, he and Sun both participated in trips to sketch at Huangshan, but their behaviour created gossip. Jiang Biwei, mother to Xu's two children, was so annoyed that she destroyed all of her husband's work that featured Sun. The controversy resulted in Sun leaving the university, but Xu Beihong did not leave his family. The status of their affair has been questioned in later accounts, as Xu's second wife, Liao Jingwen, wrote in her biography that was never any improper relationship between Sun and her husband. Instead, Liao stated that the gossip was mainly spread by Zhang Daofan, who wanted to separate Jiang Biwei from her husband. Sun returned to her old school and began teaching but without her degree. Xu and his partner finally agreed privately to separate in 1935. Sun published her first book Sun Duoci Sketches in 1936.
The outbreak of the Second Sino-Japanese War necessitated in 1938 that Sun and her family should move to Changsha and here she again met Xu. Xu arranged for her whole family to move to Guilin where she worked as a civil servant. Xu, who was now a nationally well known painter, announced publicly in the newspaper Central Daily News that he would no longer live with his wife although it appears that they were never formally married. Xu sent a go-between to ask her father for permission to marry. Her father refused and much to Sun's later regret she decided to accede to her father's wishes. Sun and her family moved again to Lishui, Zhejiang province, where the 26-year-old Sun entered into an unhappy marriage with the much older and unfaithful provincial education minister Xu Shaodi. Sun taught at Zhejiang College of Art. Sun continued to correspond with her former lover and Xu Beihong was said to have memorised poems that he sent her.
In 1948 Sun and her husband moved to Taiwan where she taught at the art department of National Taiwan Normal University. She later moved to the United States, where she heard of Xu Beihong's death in 1953. Sun mourned his death for three years. She died in Los Angeles in March 1975 from cancer.