Abstract:  In order to solve the problem of the deviation of the total boron concentration of the coolant of nuclear island primary circuit measured by the off-line boron meter (OFBM) from the chemical titration boron concentration in the primary coolant in pressurized water reactor nuclear power plant exceeds the standard, the boron concentration algorithm of the OFBM was analyzed, and the impact factors and causes of the boron concentration deviation were analyzed. The analysis results show that the main impact factors of the boron concentration deviation were the 10B concentration and the 10B abundance of the nuclear island primary circuit coolant; the main reason of the boron concentration deviation generated was the OFBM boron concentration algorithm ignoring the 10B abundance change in the primary circuit coolant of the nuclear island. A new boron concentration algorithm capable of tracking the 10B abundance of the coolant of the nuclear island primary circuit was designed at the same time, and the 10B concentration calculation function was implemented in this new algorithm. The simulation model of the new boron concentration algorithm was built finally, and the calculation accuracy of the new boron concentration algorithm was verified based on the calibration data of OFBM of two M310 units. The simulation results show that the calculation accuracy of the new boron concentration algorithm meets the OFBM design specifications.