In 2019, 13th Standing Committee of the National People's Congress appointed him Chinese Ambassador to India, replacing Luo Zhaohui.

[7][8] In an interview with the Press Trust of India on 25 June 2020, Sun stated that “mutual respect and support is a sure way and meets the long-term interests of both countries; suspicion and friction is a wrong path and goes against the fundamental aspiration of the two peoples” and then proceeded to blame the Indian side for instigating the border clash stating the onus to resolve border tensions was not on China.

[12] During his farewell ceremony, he stated that “if the western theory of geopolitics is applied to China-India relationship, then major neighbouring countries like us will inevitably view each other as threats and rivals” which was seen as a cautionary remark against the West.

[14] That same month in a meeting in Beijing, he warned Cagayan governor Manuel Mamba on hosting foreign troops such as the United States in the Philippines.

During his meeting with Choe, both China and North Korea pledged to strengthen strategic communications "at all levels" and reaffirmed "unswerving stance" on deepening ties.

[17] Sun is married to Bao Jiqing (包吉氢), who is a distinguished researcher at the Center for South Asian Studies in Institute of International Relations at Tsinghua University, and worked as a teaching assistant and lecturer at the English Department of China Foreign Affairs University from 1991 to 2001.