In 2007, Sun started dating Liang Zi (亮子), a photographer. They married in 2017.

Sun started her modeling career in 2008 when she represented China in Elite Model Look, one of the world’s most influential modeling contests. She won the championship in China and eventually won the third prize internationally. She made her international runway debut during London Fashion Week in February 2010, walking for Christopher Kane. In December 2009, she was handpicked by Karl Lagerfeld to walk Chanel’s Paris-Shanghai fashion show.

In covers and editorials, Sun first appeared in the Vogue Italia editorial in January 2011, The Power of Glamour, shot by Steven Meisel. Later, Meisel also enlisted her for editorials in other major magazines, including Vogue US, V, W and others.

In model campaigning, Sun’s first major campaign was Dsquared2 Spring/Summer 2011. Since then, she has also been the face of Prada, Louis Vuitton, Lane Crawford, Calvin Klein’s cK one, Chanel Beauty, Giorgio Armani Beauty (3 seasons consecutive), CK Calvin Klein beauty, Barneys New York, Diesel, DKNY Be Delicious fragrance, H&M, Prada and most recently Christian Dior and Chanel Watches. In spring 2012, Sun became the first Asian face of Italian brand Valentino. She was selected as the face for Chanel Beauty, Calvin Klein cosmetics, Giorgio Armani beauty and Dolce and Gabbana skincare.

In 2013 she became the first Asian model to front a solo cover for Vogue Italia’s January issue. She was the first Asian model to front Valentino campaign. In 2014, she became the first East Asian model on the fold cover of the September issue of American Vogue. In 2017, she became the face of cosmetic giant Estée Lauder.

In 2014, she was featured alongside eight other models on the cover of the September issue of American Vogue, becoming the first East Asian model on the fold cover of American Vogue. She has also been the cover girl of Vogue China, Vogue Japan and i-D and has appeared in editorials of Vogue Paris, British Vogue, Vogue (USA) among others.

In rankings by Models.com, Sun is first listed on the Top 50 Models Women List by Models.com in 2014. In 2016, she was moved from the top 50 Models to the list of Industry Icon, a status reserved only for models with high impact and longevity in the industry. In 2017, Sun again was moved to the New Supers and Money Girl List, solidifying her as one of the top faces in modeling.

She made a second appearance on the cover of Vogue Italia September 2014 edition and also made history as the first East Asian model to appear on the fold cover of American Vogue for the September issue 2014. In June 2015, Fei Fei made a third appearance on the cover of Vogue Italia.

In 2017, she once again fronted Valentino Spring 2017 advertising campaign in addition to campaigns by Theory, Tory Burch and Massimo Dutti. In June 2018, Sun graced the cover of Allure magazine, along with Fernanda Ly and Soo Joo Park. In 2019, she made her appearance on the debut and launch cover of Vogue Hong Kong.

She was notably the showstopper of the Vera Wang Spring/Summer 2020 fashion show, held on September 10, 2019 as part of the New York Fashion Week. However, she fell down twice in a row during the finale because of the designer’s very high heels paired with a slippery catwalk.