Abstract: The establishment of the simulation model of the volume compensator is significant for the simulation of operation of 200 MW low-temperature heating reactor. In the paper, the water region of the volume compensator was divided into water fluctuation region and main water region, and three regions, nitrogen region, main water region and water fluctuwaation region occur in the simulation model of the volume compensator. This three region model is different from two region model available. The behavior of the nitrogen in the volume compensator was simulated by polytrophic process similar to real variety process of gas, or constant temperature process of gas with the valve opening in the volume compensator and adiabatic process of gas with the valve closed in the volume compensator, respectively. The former were closed relatively to real variety process of gas space nature. The differential equations were solved by using Gear method. The calculated results show that the model presented in the paper can simulate the behavior of the volume compensator well. This study provides a valu- able method for the design and safe operation analysis of the 200 MW low-temperature heating reactor.