The vibration characteristics of an underwater pneumatic launcher model were studied. Shorttime Fourier transform method was used to analyze the radial acceleration signals of the launching tube. Combined the interior ballistic process, it was shown that the vibration signals contained two parts. Part 1 was mainly caused by the inflating of the launch tube，the cuttingoff of the sealing metal film and the starting to move of the weapon. A main vibration frequency of 1 400 Hz was found with high frequency components reaching about 5 000 Hz and a duration time of about 0.038 s. Part 2 was caused mainly by the emitting of the weapon from the launch tube and the deflating process of the chamber pressure, with a main vibration frequency of 1 100 Hz and a duration time of 0.016 s.The energy spectrum of wavelet packet transform indicated that the energy percentages of part 1 and part 2 were 79% and 10% of the total, respectively. Therefore part 1 of the vibration signals has main contribution to the whole vibration of the launching process.