To calculate the system capacity accurately in environment test equipment design, a simulation scheme was introduced. This scheme was based on distributed parameter model and the model was expressed in state-space. In the traditional design, refrigeration of loading was regarded as completeness atthe end of refrigeration course. As a result, the system capacity is calculated much greater than the actual need. Compared with traditional design, the simulation design scheme is more accurate in system capacity calculation. The simulation scheme can be applied to test and compare different control scheme too. It is of distinct meaning and efficiency in the initial design stage.