Based on numerical simulations at home and abroad,a more accurate calculation program for solving gain field of mixing gasdynamic CO2 laser (MGDL) was set up. Using this program,small signal gain distribution rule in optical cavity of mixing gasdynamic CO2 laser based on screen nozzle was studied,and the influence of inlet total temperature of donor nozzle on small signal gain of mixing gasdynamic CO2 laser based on screen nozzle was studied. Calculation results indicate that there is an optimal inlet total temperature of donor nozzle which can maximize the small signal gain in optical cavity while the other parameters keep unchanged. While the inlet total temperature of donor nozzle is lower than this optimal value,small signal gain in the same position of optical cavity becomes higher when inlet total temperature of donor nozzle becomes higher. While the inlet total temperature of donor nozzle is higher than this optimal value,small signal gain in the same position of optical cavity becomes lower when inlet total temperature of donor nozzle becomes higher. Furthermore,influence mechanisms of inlet total temperature of donor nozzle on small signal gain are analyzed.