Flexible user interface system and the effects are described for Module-based Simulation System (MSS) for complex, Large-scale simulation of nuclear power plant. MSS has been developed based on four methods of (1)module database, (2)automatic module integration, (3) knowledge-based simulation support and (4) human interface. With the introduction of MSS, even a novis user can bulid up a large complex simulation program in a short time, run the program and obtain the end results by the from of graphics, thus to improve cost, time and enviroment in human software work.