Basic frequency laser of high power operating on the condition of low-ordermode was obtained according to the design of Z shape resonant cavity which can get large volume of basic frequency. And the spot size on the Nd∶YAG stick is insensitive to the variation of thermal focal length. It is convenient to analyze the output characteris-tic of resonant cavity using ABCD matrix. In addition, 1 /4λwave plate compensating the thermal birefringence was analyzed theoretically which is in good agreementwith experimental data. In the experiment, we adop ted 9 LD bars each ofwhich has 20W output power to side pump single Nd∶YAG stick, single Q-switch, Z shape design which real-izedM2 factor 1. 2, the output power stability of the laser RMS < 1% and the 20W average output power.