護法運動
Shortly after, Yuan Shikai tried to proclaim himself Emperor, causing the National Protection War. Cheng returned to Hunan to enlist rebel soldiers in Cai E's army. During the Constitutional Protection Movement he was first appointed military commander of Changsha, then Deputy Minister of War in Sun Yat-sen's Guangzhou Government. He was put in charge of training troops in Guangzhou, and then took part in the Northern Expedition. From 1925 to 1927 he was the General commanding the 6th Army, and briefly served as Chairman of the Government of the Hunan Province in 1928. In 1926 he was elected a member of the Central Executive Committee of the Kuomintang. It was in these years that Cheng first met and collaborated with Mao Zedong, as the Kuomintang and the Communists were working together during this time under the direction of the Soviet Union, with Mao at one time serving as Cheng's assistant in political and propaganda affairs.