After the 18th Party Congress held in November 2012, Sun became the party chief of Tianjin municipality, China's richest provincial-level jurisdiction by GDP at the time, taking the post vacated by Zhang Gaoli, who became a member of the Politburo Standing Committee. As Party Secretary of Tianjin, Sun joined the elite ranks of the Politburo of the Chinese Communist Party as one of the two women on the body (the other was Vice-Premier Liu Yandong). She additionally became the first female party chief of a direct-controlled municipality in party history.