Abstract: In the analysis and design of Nd:YAG Q-switched lasers, researchers usually choose to ignore the influence of laser lower level lifetime on pulse waveform. When the laser pulse width is much larger than the laser lower level lifetime, this approximation generally does not bring much deviation. When the pulse width reaches the nanosecond level, the influence of the lower level lifetime of the Nd:YAG crystal about 30 nanoseconds on the pulse waveform will become very serious. The theoretical analysis model of the influence of Nd:YAG lower level lifetime on the output pulse waveform was established, and the output waveform of the narrow pulse width Q-switched Nd:YAG laser was simulated. The results show that in the case of narrow pulse width laser output, the lifetime of the lower energy level of the laser will lead to the tail peak of the Q-switched pulse after the main peak, and the tail peak energy can reach more than 100 % of the main peak energy. At the same time, the experimental system of Nd:YAG acousto-optic Q-switched laser was established. The Q-switched pulse waveform was measured under the condition of approximate simulation calculation, and the tail peak phenomenon consistent with the simulation results was observed. The correctness of the theoretical model was verified by experiments.