The invention relates to a method for calculating a nuclear island variable working condition heat storage increment of a pressurized water reactor nuclear power station. The method is characterized by comprising the following steps: (1) establishing an overall energy balance equation of a pressurized water reactor nuclear power unit primary coolant loop, a steam generator secondary side loop anda nuclear reactor by utilizing an energy conservation principle, and analyzing the energy balance equation to obtain a key variable influencing the nuclear island variable working condition heat storage increment of the pressurized water reactor nuclear power unit; (2) mathematical modeling is carried out on the obtained key variables, and the influence quantity of the key variables on the heat storage capacity under the variable working conditions is calculated through the obtained mathematical model; and (3) summing the calculated values of all the key variable mathematical models to obtainthe increment of the heat storage capacity of the nuclear power unit under the variable working condition of the nuclear island, thereby completing the calculation. By means of the calculation method,the problem that the nuclear power unit does not have a real machine test is effectively solved, and the calculation method capable of obtaining the nuclear island variable working condition heat storage increment of the nuclear power unit is provided.