[Objectives]In order to carry out the comprehensive thermal calculation of the marine secondary circuit systems of nuclear power plants conveniently and efficiently,[Methods] a thermal dynamic calculation method under all operating conditions is proposed and a mathematical model of a secondary circuit system established on the basis of the features of the secondary circuit systems of marine nuclear power plants, the characteristics of main steam turbine sets and accessory equipment, and the calculation results of equipment parameters and heat balance under several typical operating conditions. Several powers of main steam turbine are set evenly between minimum and maximum operating conditions, and the power of the main steam turbine is used as the input argument. The thermal parameters of the equipment change synchronously through a several-layer nesting cycle. The changing laws of such parameters as the steam consumption of each main piece of equipment, temperature of the steam and water in each heat exchanger, and overall steam consumption and efficiency with respect to the operating conditions are obtained. The reasons for the above-mentioned changing laws are analyzed and the calculation values compared with the design values.[Results] The results show that the margins of error of the system parameters are small, which satisfies calculation accuracy requirements.[Conclusions] The proposed method and changing laws in this paper provide a reference for the study of the parameter matching characteristics of secondary circuit systems.