At the direction of the Party, Kang worked underground as a labor organizer. He helped organize the February 1925 strike against Japanese companies that culminated in the May 30th Movement, a huge Communist-led demonstration, and brought Kang into close contact with Party leaders Liu Shaoqi, Li Lisan and Zhang Guotao. Kang participated in the March 1927 worker's insurrection alongside Gu Shunzhang and under the leadership of Zhao Shiyan, Luo Yinong, Wang Shouhua and Zhou Enlai. When the uprising was put down by the Kuomintang with the crucial assistance of Du Yuesheng's Green Gang in the Shanghai massacre of April 12, 1927, Kang was able to escape into hiding.