A new simplified method-compound lens method- for analysing the thermostable telescopic resonator is reported. Using this method, we suoessfully obtained the stable output with a large volume of TEM00 mode for a high repetition rate Nd: YAG laser. The output energy of the resonator at 1.06μm is 250 mJ per. pulse over the range of 1-10pps, and the output energy fluctuation is less than ±2%.