Abstract: The core cooling and monitoring system（CCMS） is installed in CPR1000 nuclear power plants to measure the subcooling margin of the coolant at the core outlet. Every error origin in the process of the measurement was investigated in this paper. The uncertainties of the subcooling margin measurement was estimated in saturation condition for the case of the distribution of the coolant temperature at the core outlet is heterogeneous or homogeneous, and the curves of uncertainty interval boundaries changing with primary pressure were obtained, based on which the method for determination of the error（ε） curve of the subcooling margin measurement which is used in CCMS to diagnose the cooling state of the core is developed. The acquired results have been practically implemented in CPR1000 nuclear power plants.