The absorption linewidth of the rubidium atom is very narrow, the pump absorption linewidth of the rubidium atom can be wider by filling a ration of buffer gas, and the absorption rate of the pump light can be also improved. The correspondence among the incident pump spectral width, incident pump intensity and the buffer gas pressure is gained, the best working conditions are also found. The high pump absorption efficiency from the center frequency for rubidium atom can be achieved, the total pump absorption efficiency within the spectral width can be also ensured. The simulation results show that with the increasing of the buffer gas pressure, the absorption linewith of the rubidium atom increases, but the total absorption efficiency of the pump light decreases unobviously.