The factors that deteriorate beam quality of repetition DPL include maladjustment of resonator, un-even distribution of pumped light and thermal effect of working medium. Directional prism is as entirely reflect mirror which builds up critical resonator with piano output mirror, and it realizes symmetry gain in side pumped cavity, optical compensation to thermal effect and optical axis self-correction. Directional prism resonator works in Conductive cooling, side pumped DPL with frequency of 40 Hz, experiment results are as follows: 0.532 μm laser energy is of 53.2 mJ, stability of output energy is of 0.5%, pulse width is of 7.5 ns and the stability of frequency is of 0.025%, original beam divergence is of 2.8 mrad, laser axis is nearly nil-drift in two hours, and the beam distribution in far field is steady.