Abstract: In order to evaluate the performance of the gain generator in a chemical laser, the small signal gain has been measured using the variable output coupling method. The stable resonator was composed of two mirrors, one concave mirror and one outcoupling mirror. The ternary coupling problem about saturation intensity, non-output wasting and small signal gain in measured data was translated into a dualistic numerical problem. Processing results, which met engineering requirements, were derived through numerical fitting, and the errors in experiment and data processing were analyzed. Through the variance analysis, it was found that the fitting curves of the average two accorded with experiment results better than the fitting curves of the average one. There existed a maximum output power in the