Abstract: The thermal test study focusing on air cooling performance of the control rod drive mechanism（CRDM） for ACP100 is carried out in the environment of practical reactor operation condition, which is simulated as 1:1. The balance temperature of the CRDM in different cooling air speed condition, the changing of the temperature after the cooling air broken off, and the electrical characteristics were measured. Some crucial data such as the minimum cooling velocity, the allowable time and the maximum performance without cooling were acquired. The test results provided an important reference for the cooling system design and operation of the small modular reactor ACP100.