The dependence of slowing light on the saturation effect in the vertical-cavity surface-emitting laser (VCSEL) is demonstrated. Combining the boundary conditions with the optical field distribution in VCSEL, the phase expression and the delay expression of the signal are derived. Based on these expressions and the rate equation, the relation between signal power and time delay is presented. Then the experimental demonstration shows the delays and the eye diagrams of a 2.5 Gbps pseudo-random binary sequence (PRBS) signal, when the VCSEL is working in the saturation condition. The theoretical and experimental results show that the high-power signal transmitted in the VCSEL could achieve better signal quality at the expense of the lower time delay. In order to obtain larger delay, on the one hand, the signal power should be controlled. On the other hand, the signal wavelength should be carefully adjusted to track the peak gain wavelength of the VCSEL caused by the saturation effect.