Nuclear Power Plants are complex systems. They are essentially composed by many components of rather few types. By adopting Modular Modelling techniques, a package-Nuclear Power Plants' Modular Modelling and Simulation System (NPPMMSS) is developed, which is a flexible and versatile modelling-and-simulation tool for the researchers and modellers of nuclear power plant. This paper describes the NPPMMSS' framework, features, applications and the methods of model selection, model interface, modular model building, model connection etc. Finally, a case study is shown.