Abstract: No.2 shutdown system of CANDU-6 heavy water reactor in Qinshan III Nuclear Power Plant tripped several times due to the failure of the regional power monitor loop. Typical three channel trip failures were analyzed. The amplifier and potentiometer in the regional power monitor loop were analyzed theoretically and practically. Potentiometer failure contributed to the channel trip. Abrasion of contact patch and resistance thread because of long-term adjust is the main reason of potentiometer, according to the analysis of the failure potentiometer. Maintenance strategies such as periodic check and potentiometer replacement are developed, and the reliability of the regional power monitor loop is improved.