The cooling characteristics of night ventilation and control method were studied.Two rooms with different functions in Changsha were selected as the research objects,which were simulated by DeST,respectively.By analyzing the relationships among ventilation time,air exchange frequency and exhaust temperature,the cooling characteristics of night ventilation for different buildings can be obtained.The results show that the exhaust temperature varies logarithmically with ventilation time.A method to determine the optimal combination between air changes and ventilation time under taking the same waste heat of room can be obtained when the optimal ventilation energy efficiency is taken as the goal.By using this method,the optimal control of the ventilation system operation can be achieved for the highest ventilation energy efficiency.The method and the results are available for the optimal design and operation of the night ventilation system.