In order to study the effect of vibration on friction at contact interface, an experimental device was designed. The slider sliding at uniform speed on the vibrator was taken as the object. The mechanism of friction reduction caused by normal vibration and tangential vibration at interface was analyzed by means of experiment and theory. The experimental results show that normal vibration and tangential vibration often exist simultaneously. When the interface vibrates and the direction of the tangential relative velocity between the slider and the vibrator remains constant, the average sliding friction force of the slider is the same as that without vibration, and there is no friction reduction effect. When the interface vibrates and the tangential relative velocity direction changes periodically or stick-slip, there is an obvious friction reduction effect compared with that without vibration. Coulomb’s friction law was used to establish a dynamic model for simulation, and the simulation results are in good agreement with the experimental results, indicating that Coulomb’s friction model could reflect the friction between interfaces under the condition of known interface motion. The experimental and theoretical results show that the main reason for friction reduction is the periodic change of friction direction between interfaces or stick-slip caused by the tangential relative motion due to the vibration, when normal vibration and tangential vibration exist at the same time.