All-optical atomic magnetometer with high sensitivity is an important technique to detect weak magnetic field. The Cs vapor cell is the core component of the atomic magnetometer. He gas is used as buffer to reduce diffusion to the walls where spin coherence is rapidly lost. The buffer gas decides the pump light frequency. In this work, we analyze the principle of all-optical Cs atomic magnetometer and the influence of pump light frequency on the sensitivity of all-optical Cs atomic magnetometer as the buffer gas pressure is about 1.333×104 Pa. Rate equation is used to calculate the orientation of the atomic spin for different pump light frequency. The result shows that the maximum sensitivity appears as the pump light frequency and the probe light frequency are locked to Cs D1 transition F =3 → F′ = 4 and Cs D2 transition F =4 → F′ =5，respectively.