Abstract: Anderson's relaxation model of two modes and three vibrational temperatures was combined with Yan Haixing’s vibrational relaxation data to simulate 2D non-equilibrium nozzle flows in a gasdynamic laser. Two different minimum-length nozzles with supersonic area ratios of 50∶1 and 20∶1 were studied by numerically solving the conservational equations. The small signal gains of the simulation results agree very well with J. S. Vamos's experiments for both nozzles. This analysis method solves the problem of matching the non-equilibrium nozzle flow analysis results of gasdynamic laser nozzles with the experimental data, which conventional quasi-one-dimensional simulation cannot do well. By this way, highly accurate method of evaluating a gasdynamic laser nozzle’s performance was realized.