Floating nuclear power plant (FNPP) has the advantages of high efficiency, flexibility and economy. It can provide energy to remote islands and power to offshore oil and gas exploration platforms. However, the power grid load of FNPP may experience significant fluctuations in the short term due to factors such as weather changes and equipment failures. As a highly coupled nuclear-thermal-electric system, the large-scale fluctuations of grid load may have an impact on the stable operation of both primary and secondary circuits of the nuclear power system. In existing studies, the nuclear-thermal energy system and the electric power system usually use different simulation tools, and the simulation is relatively fragmented, which makes it difficult to verify the stability of the operation of FNPP. This study leverages the advantages of multidisciplinary modeling using Modelica to develop a co-simulation model for the pressurized water reactor and the electrical system of FNPP. Based on the simulation model, the transient response of the nuclear power system's primary and secondary circuits is analyzed under large-scale power grid load fluctuations so as to validate the operational stability of FNPP.