A theory for laser resonators with low sensitivity to thermal effect was introduced, the new analytic solution was obtained. The influence of the changing of the cavity parameters was analyzed and some important results were achieved. A monomode Nd: YAG laser was designed by our procedure and our theory was tested and verified through experiments. From the experiments, we found that the laser can operate at two different stable zones which have unequal threshold, and also that for a reasonably designed resonator, its operating state with. stability to thermal effect corresponds to the optimum TEM mode output.