Abstract: The paper introduces the design of pneumatic circuit for the personnel airlock in nuclear power plants, and presents an optimal design by considering the potential failure modes of the personnel airlock. In terms of the overall accidental tightness failure and the accuracy of the partial tightness detection, the tightness detection procedure of the personnel airlock has been improved. The optimized design of pneumatic circuit and the improved tightness detection procedure have been successfully applied in the third generation nuclear power reactors under construction. It ensures the great enhancement of the operational safety of personnel, and at the same time, the possible tightness failure of the personnel airlock which may result in the loss of integrity of the third safety barrier is avoided.