Lu Dingyi joined the Communist Party of China in 1925, while he was studying electrical engineering at the Nanyang Public School. After graduation, he fully joined revolutionary activities, being mainly involved in the Communist Youth League, writing articles for its newspaper Chinese Youth (later renamed Proletarian Youth and then Leninist Youth). In 1927 he took part at both the 5th CPC National Congress and the CYL Congress, being elected a member of the CYL Central Committee working with its Propaganda Department. He was actively involved in countering Chiang Kai-shek's anti-communist coup, organizing communist unities in Guangdong. In 1928 Lu Dingyi took part at the 6th CPC National Congress and the CYL Congress, both of which were held in Moscow, remaining in the Soviet Union until 1930 as a junior representative of the CYL to the Comintern.