Abstract: Considering the problem of serious aging, high cost for maintenance and upgrade of the original first loop hydrostatic test overpressure protection device of the Daya Bay nuclear power plant, a new digital overpressure protection device for the nuclear plant is designed to replace the old one, which is based on the Siemens S7-200 PLC system, and combined with the analog input module EM231, analog output module EM232 and human-machine module TD400 C, the new device can achieve dynamic real-time monitoring, analyzing and processing of the pressure data and simultaneously displaying of all the data, and output the control signal after logic operations on the test data during the hydrostatic test. Meanwhile a new offline test platform for the overpressure protection device is built to do offline testing and calibration. The new designed overpressure protection device is with advantages of more flexible, portable, high-precision, smart and easy maintenance. And the new designed equipment has been successfully used in Daya Bay and other nuclear plants for more than two years, to make a great contribution to shorten the overhaul period.