Aiming at slow light phenomenon in the thin film, the mechanism of slow light and its influence factors are investigated. Because the thickness of the thin film is extremely small, an assumption that the component of the magnetic field in the thickness direction is independent of the thickness is proposed. Then the dispersion relation in the thin film media is obtained. The expression of the group velocity of the light in the thin film media is deduced. Then the influences of the frequency of the light, the refractive index of the thin film media and the thickness of the thin film media on the group velocity are analyzed. The influence curves of the group velocity in the thin film media are acquired. The research shows that speed of the light in the thin film media can be greatly reduced by selecting the appropriate influence factors. It provides a theoretical basis for the slow light technology in thin film media.