Mechanical ventilation combining with free cooling or air-conditioning (defined as combined-ventilation) in summer and transient seasons can effectively decrease the energy consumption of air conditioning systems for buildings. It’s difficult or even impossible to optimize the combined-ventilation rate by using traditional method, direct simulation method. Due to this, some new concepts—optimization of combined ventilation strategy based on an inverse problem to determine the optimized combined-ventilation rate was proposed by us. They can be used to optimize the mechanical ventilation rate so that the total energy consumption of the air-conditioning system in the building can be minimized. A two-plate model was adapted to simplify the thermal performance analysis of a real room, which was validated with the simulated results of DeST-a software widely-used building thermal performance in China. A kind of nonlinear optimized method, the Sequential Quadratic Programming (SQP) algorithm, was applied to solve this nonlinear problem. For illustrative purpose, the instantaneous combined-ventilation rate of a room in a building in Beijing in summer was optimized and a sensitivity analysis was conducted. It concludes that from the example: (1) The energy consumption can be decreased by 52.04% by optimization-the peak cooling demand can be decreased 45.89 % and the operative hours of an air-conditioning device can be reduced nearly 10 hours; (2) Optimized ventilation rate strongly depends on the temperature difference of mean walls’ surface and outdoor temperatures-a correlation between these two variables has been derived to determine the optimized combined-ventilation rate; (3) The energy saving potential for a building in a given climate area depends upon the so-called Effective Energy Efficiency Index. Based on the analysis, several suggestions are presented.