In order to identify the relationship between main reaction and key parameters, we use the sensitivity analysis method to study the reaction mechanism of HBr chemical laser. The sensitivity coefficients of temperature, pressure and key intermediate product are analyzed, and the effectiveness of temperature on controlling the distribution of HBr is confirmed. A gain generator of HBr chemical laser is studied by computational fluid dynamics, and the distributions of temperature and concentration are given, respectively. The research results show that high vibrational state is obtained at the initial temperature of 500 K.