Night ventilation is an effective measure to decrease the air conditioning load in summer night. However, for mechanical ventilation, the proper trade-off between the decreased air conditioning load and the increased fan energy consumption should be balanced. The climatic cooling potential of 204 major cities in China was evaluated. In order to make full use of the climatic cooling potential, a new method based on the inverse problem to determine the optimal night mechanical ventilation strategies was proposed. As illustrative examples, the optimal mechanical night ventilation strategies of the whole summer of the typical year in Beijing was investigated. The total electricity energy consumption of the whole summer has been decreased from 210.22 kW·h to 100.53 kW·h. The optimal ACH is a function of the instant indoor-outdoor air temperature difference and the air conditioning load of today and the next day. The energy saving ratio of optimal ventilation strategies increases with larger thermal conductivity and capacity of internal wall, however, that of external wall has little influence on the optimal energy saving ratio. The proposed method can also be used to deal with other similar nonlinear optimal control problems in the built environment field.