Abstract: Operating characteristics of the heater affect the stability of regenerative system and thereby the performance of the whole thermal system. To understand the operation of the heater better, according to the different working fluids in the heat exchanger, using the modular modeling method, the mathematical models for single phase heat transfer section and steam liquid two phase section of the heater were established, and then a common surface type transient mathematical model of the heater is obtained. Separate modeling for the two types of heating surface relative to that of the condensate section to replace the whole heater, the precision is improved. Through a combination of the two types of heat transfer model, the high pressure heater and the low pressure heater models were obtained. Taking a high pressure heater as an example, the simulation is applied to analyze changing progresses for working fluid flow, temperature and pressure under feedwater flow changes. This study shows how the change of feedwater parameter affects the heater's performance.