Abstract: Organic Rankine Cycle have numerous potential applications in thermal power generation at medium-to-low temperatures. Heat exchanger is an important part of ORC, but the research on the off-design conditions performance of the heat exchanger is not enough. A simulation modeling approach of the heat exchanger under off-design conditions is proposed in this paper and the simulation model of the heat exchanger is established and verified by experiments. According to the verification, the maximum error of heat transfer quantity between simulation and experiment of evaporator and condenser is 3.74% and 3.20% respectively. On the basis of the model validation, the performance of the heat exchanger in ORC are evaluated, the off-design conditions performance and the thermal transfer area migration regulations of the heat exchanger are obtained, and the equivalent heat transfer coefficient of the heat exchanger is defined and obtained, which provides guidelines for the design of the heat exchanger and the application of the device in ORC.