How to stabilize the pressure of the large space in the comprehensive experiment of high/low temperature environment simulation system is a critical problem, to meet the test requirements, the expert PID system control algorithm is studied. This paper introduces high/low temperature environment simulation system, references to the control method of expert system, establishes the knowledge base and reasoning machine, simulates expert knowledge reasoning, and compares the used expert system control with the unused expert system control effect. At present, the system has been successfully completed test, passed the expert examination and put into operation. The running results show that the algorithm can maintain in micro positive pressure and control cabin pressure stability, reached the test requirement.