The expression for output laser intensity of solid state laser under unstable state is deduced in theory. And the gain characteristics of Nd:glass heat capacity laser is stimulated on the basis of that expression. It is shown that high pumped intensity, large emission cross-section, high aspect ratio can increase the extraction efficiency. The influence of temperature on extraction efficiency can be ignored under high pumped intensity. In addition, the transmission ratio of cavity mirror has an optimum value because of the influence of temperature on the output of the pulse string.