A new method identifying the modal parameters of diesel engine based on the dry running is proposed. According to the analysis of its excitation law, by increasing the rotating speed of diesel engine, the larger the gas impact sequence density and the shorter the gas percussion time generated by the combustion chamber with a certain space, the larger the obtained excitation source and the wider the frequency band. The dry running self-excitation test is conducted, and its results are compared with the identification results of traditional percussion test. The identification results of two states are very similar, and the maximum deviation in natural frequency identification results is 1.9 Hz. The deviation may be due to the fact that the percussion test is conducted in the static state of diesel engine, while the dry running self-excitation test is conducted in the operational state of diesel engine. The accuracy identifying the modal parameters of diesel engine based on the dry running self-excitation is verified by the test.