A four-level system is analyzed as an example,and the necessary minimum diffraction efficiency of acoustooptical Q-switched laser in a certain output pulse energy is studied. Through soluting the population inversion density , the relationship between a certain output pulse energy and cavity loss,the necessary minimum diffraction efficiency of Q-switch , the reflectivity of output mirror and so on,are acquired. The result can be used to design the diffraction efficiency of acoustooptical Q-switched laser.