Because of the great resources potential and the feature of low pollution of gaseous fuel, gaseous fuel internal combustion engines (gas engines) have been paid more and more attention. However, their average thermal efficiency is just about 30-40% wasting a huge amount of energy by exhaust, cooling water and so on, so waste heat recovery is very meaningful. Both the RC (steam Rankine Cycle) and the ORC (Organic Rankine Cycle) are regarded as the suitable way of WHR (waste heat recovery) for internal combustion engines. Therein, RC is usually used in large engines. The WHR system is always designed at rated work condition, while the gas engine may often work at different conditions. This makes the property of the waste heat source change, which affects the performance of WHR system, so it is very important to research its performance at variable working conditions. In this paper, a stationary electric generating natural gas engine of rated power 1,000kW is the studied object and RC is used as WHR system to recover the exhaust waste heat. A dynamic math model of this RC WHR system which takes into consideration of the interrelationship among main components is built by Simulink software. With this math model, not only the dynamic performance but also the steady state performance of the RC system can be calculated. Finally, the evaporating pressure, absorbed heat, output power and other parameters of RC at steady state varying with the property of exhaust is analyzed, which can provide references for the engineering application.