A kinetics model including five levels is presented according to the emission spectrum of Cu atom observed from a CuBr laser plasma. The numerically calculated laser power, peak time and weveform, by employing the experimental data of electron temperature and density, well conform to our measurement. At fixed input power, it is also shown that to change the difusion coefficient of the buffer gas will change the average temperature of the gas or the initial density of Cu atom, therefore, change the laser power as well.