It is an important task to study the passive Q-switching used in DPL high power pumped. The theory of the passive Q-switching is discussed. When DPL was pumped by high power, the characteristics of output plus series were evaluated as the function of the argument T_0, which was small-signal transmittance. The results are shown by the curves that plus width and repetition rate increase and peak power decreases,when small-signal transmittance increases. The comparison results of passive Q-switching and one of A-O Q-switching show that passive Q-switching is applicable to DPL high power pumped.