As a junior Sun participated at the 2014, 2015, 2016 and 2017 World Junior Speed Skating Championships. At the 2015 World Junior Speed Skating Championships she won the silver medal in the team sprint event and the bronze medal in the 500 metres event. Two years later at the 2017 World Junior Speed Skating Championships she became junior world champion in the team sprint event and won the silver medal in the 500 metres event. Nan represented her nation at elite level at the World Sprint Speed Skating Championships for Women in 2017, finishing 23rd overall. She also competed at other international competitions, including at ISU Speed Skating World Cups.