This paper described the principle of an all-optical cesium magnetometer based on absorptive detection. In order to reduce transverse relaxation rate and to maximize spin polarization time of the alkali-metal atoms, it is usually to fill the inert gas He and the diatomic molecule N2 which are used as buffer gases into the cell to achieve high measuring sensitivity. Not only the collision probability of polarized atoms with the cell wall but also the radiation trapping can be reduced or avoid by this approach. The relationships between the output signals of this magnetometer with buffer gas pressures were expressed here. After a detail theoretical analysis, it was found that the optimal gas pressure of the buffer gas was about 3.9×104 Pa for helium (He) and 3.6×103 Pa for nitrogen (N2).