Abstract: Based on the principle of pressure decay leak detection, and a comparison of the experimental and theoretical analyses between the pressure decay leak detection and the leak detection under non-vacuum environment, the phenomenon of pressure increase during the detection of large-sized sealed vessel is analyzed. It is shown that the pressure increase is due to the fact that the real leak rate is less than the sensitivity of the pressure decay leak detection. In the detection cycle, the extension of dwell time could be effective for an accurate detection.