Emergency diesel engine is the last safety barrier for nuclear power plants. Nuclear safety criteria require that it can start up to rated speed（ such as 1 500 r/min） within 10 seconds in an emergency and have loading conditions,so its startup performance is particularly important. During the start-up process,the power of high-pressure air is a key factor for success.In this paper,a vibration-based method by analyzing high-pressure air（ domestic units are generally 40 bar·g） power to evaluate starting performance is presented. The large amount of monitoring data obtained indicates that high-pressure air is rapidly charged into the cylinder to generate high-frequency（ approximately 11 kHz） gas exciting impact,and the impact phase will continue to shift backward with increasing speed. Firstly,the Teager energy operator and an adaptive threshold impact location method were used to automatically capture the high pressure gas impact area of each cylinder in each cycle of the start-up phase. Secondly,the energy of the local impact envelope was extracted to characterize the function of the high-pressure air. Finally,the gray evaluation method carried out a comprehensive assessment. The actual test result verifies the effectiveness of this method and has the value of popularization and application.