The shortening of development period with keeping product quality of an air-conditioning system is important issue to adopt diversified and changeable market. The control software which combines elements of the air-conditioning system has a close relationship with rework in the later development stage. To improve the control software development process, Toshiba has developed a simulation based testing platform for control software of the air-conditioning system using hybrid system modeling language DCML^<TM> (Dynamic Constraint Modeling Language). This paper clarifies the criteria of the control software, the goal of the testing platform, building steps of simulation based testing platform. And this paper reports an experience on applying the building steps to a trial environment of the testing platform.