Abstract: The determination of time discrimination and frequency discrimination and the selection of a window function in the short-term Fourier transform(STFT) were discussed to analyze the vibrational signals from a high-pressure hydraulic pump unit. This method based on the STFT overcomes the shortcoming of the fast Fourier transform. The analysis result shows that the application of time-frequency analysis method to the start-up process analysis of centrifugal type of high-pressure hydraulic pumps can not only exactly determine the change of each individual frequency in the process, but also find out some external interfering signals to judge whether a pump is working normally or not and the reason for its fault.