The invention discloses a system and a method for overtemperature control on a platen superheater of a supercritical boiler. The system comprises wall temperature sensors installed on the pipe wall of the platen superheater, an in-situ measurement device for receiving the signals of the wall temperature sensors, a power station decentralized control system DCS in communication connection with the in-situ measurement device, a first-stage monitoring loop and a second-stage monitoring loop which are connected with the in-situ measurement device and used for monitoring the wall temperature sensors, an over-fire air baffle controlled by the power station decentralized control system DCS and connected with the first-stage monitoring loop, and a desuperheating spray control valve controlled by the power station decentralized control system DCS and connected with the second-stage monitoring loop, wherein the first-stage monitoring loop comprises a first-stage speed-limiting module, a first-stage amplitude-limiting module, a first-stage OR operation module and a first-stage output pulse module; and the second-stage monitoring loop comprises a second-stage speed-limiting module, a second-stage amplitude-limiting module, a second-stage OR operation module and a second-stage output pulse module. The invention further discloses a control method of the system. The system and the method have important significance for improving the running reliability of a thermal power plant, prolonging the life of key equipment and lowering the maintenance and repair costs.