In the absorption spectrum of the medium, a burned hole is appeared by coherent population oscillation, and the linewidth of this feature is approximately the inverse of the ground state population recovery time. Based on the theoretical analysis of gain, the effect of absorption on the different pump powers is obtained. In the medium absorption range, the oscillation leads the pulse to experience absorption saturation and propagation delay; in the medium gain range, this effect induces the pulse to experience gain saturation and propagation advance. Making use of coherent population oscillation we can control the group velocity of light propagation in erbium-doped optical fiber. According to coherent population oscillation and gain theory, the analytic expression of the group index from rate equation is obtained. In the experiment the slowest group velocity was 3.45×102 m/s, and the corresponding group index was 8.7×105. The simulation results are quantitatively coincident with the experimental data.