The vibration signal of diesel engine is typically non-stationary and time-varying signal, with complicated transient components. With adaptive concentration method controlled by an additional parameter m, an adaptive generalized S transform (AGST) is developed in this paper. The simulation shows that the AGST method has a better time-frequency resolution than S transform. The AGST method is then applied to analyze the vibration signal of the cylinder head cover of a four-cylinder diesel engine. The result indicates that the time-frequency representation of the vibration signal obtained by AGST has a good resolution, and the vibration components, such as combustion shock and valve-slap, can be exactly identified. The research is of great significance for the structural optimization and vibration control for the diesel engine.