When Japan invaded China in 1937, he organized a guerrilla force to fight the resistance war in Shandong, and was the wartime governor of the province.

He was especially interested in foreign languages, and in April 1917 his translation of an article on money by Samuel Smiles was published in Chen Duxiu's influential magazine New Youth.

[1][4] He Siyuan returned to China in 1926, and was hired by his friend Fu Sinian as an economics professor at the newly established Sun Yat-sen University in Guangzhou.

[6] While at Sun Yat-sen University he published the books A Brief History of Politics and Diplomacy of Modern China (中国近代政治外交略史, 1927) and Research Methods in Social Sciences (社会科学研究法, 1928).

On 31 December, the Italian authorities arrested Yiwen and the children and handed them to the Japanese, who held them as hostages and demanded He Siyuan's surrender.

He refused the demand, condemned Japan and Italy's breach of international law through media and diplomatic channels, and held Italian missionaries in China as a bargaining chip.

[6] In November 1946, General Wang Yaowu was appointed Chairman of Shandong[10] and He Siyuan became Mayor of Beijing (then known as Beiping),[4] succeeding Xiong Bin.

[6] During the Chinese Civil War between the Kuomintang and the Communists, He Siyuan clashed with Chiang Kai-shek, who after an attempt in April 1948 to assassinate him in downtown Beijing failed, fired him in June[4] and replaced him with Liu Yaozhang.

[4] In the early morning of 18 January 1949, two bombs exploded in his home, killing his 12-year-old daughter Lumei and gravely wounding his wife, who never fully recovered from the injury.

[4][7] Undaunted, he proceeded with the negotiation and reached an agreement for the peaceful surrender of Beijing, ensuring the safety of its millions of residents and the preservation of the architectural heritage of the ancient capital.

Fluent in English, German, French, and Russian, he devoted himself to the translation of foreign works into Chinese, publishing 16 volumes by the early 1960s.