This paper proposes a first principle dynamic model of the Nuclear Steam Supply System (NSSS) for the Double-Reactor and Multi-Turbine Nuclear Power Plant (DMNPP). The main devices within the nuclear steam supply system, such as Once Through Steam Generator (OTSG), Steam Main Pipe (SMP)and Steam Turbine (ST) are modelled using the movable boundary approach and discrete approximate distributed modelling method. Dynamic behaviour analysis is then carried out on the developed model, through which, the dynamic characteristics of key variables, such as the distribution pressures in the SMP are investigated in detail. The simulation results show that when the opening degree of the ST regulating valve or the OTSG steam valve change, the fluctuations of the pressure at each point in the SMP are different. The maximum fluctuation value of the pressure is 0.12 MPa when the opening degree of the third ST regulating valve is increased by 20 % and is 0.015 MPa when the opening degree of the second OTSG steam valve is increased by 10 %. It is discovered that the steam pressure of the SMP has a self-stabilizing ability and the NSSS of the DMNPP is of highly nonlinearity and strongly coupled among multi-variables.