Abstract: The improvement of mathematical model of the pressurizer for a PWR nuclear power plantsimulator, located in Beijing Nuclear Power Plant Simulation Training Center, is presented. Theoriginal model is a revised equilibrium model. It couldn' t satisfy the requirement of simulation fi-delity for the fast transients. The improved pressurizer model is a three region, non-equilibriummodel with bubble rise and condensate drop dynamics, taking into consignation the spray and con-densate on spray heat and mass transfer processes in the pressurizer. The model was implanted inthe system and real-time simulation was realized, and satisfactory simulation effect was obtained.In addition, study on the sensibility of the simulation results to the computational time incrementis also performed.