The theoretical analysis and experimental study of the instability phenomenon of a Nd: YAG Q-switched short pulse laser was demonstrated. During the operation of the short pulse laser, there were many factors that can cause the instability phenomenon. During the experiment it was found that the interference pulse which can enter into the cavity though the output mirror can cause the instability of the short pulse laser. Based on the basis of thory, the influence of external interference light on the output light characteristics of short pulse Q-switched laser was simulated and verified by experiments.The characteristics of energy and pulse width were discussed respectively. The results show that the greater the energy of interference pulse is, the more serious the instability phenomenon is. When the interference pulse energy is 60 mJ, the loss of output energy of the short pulse laser can be more than 15%. The pulse width is 1.15 times of the original, the instability phenomenon is also more serious at the pump end of the short pulse Q-switched later. When the input energy is 60 mJ the pulse width is 10 ns, the loss of output energy of the short pulse Q-switched laser reaches 80% of the original.