In this paper, we calculate the data of hourly values of Radon and Mercury of fault gas at Wushan by means of the optimal spectrum analysis method of tidal wave. The results show that semidiurnal and daily wave components which are consistent with the theoretical earth tide exist in hourly value sequence of Radon and Mercury content of fault gas. This proves that the change of Radon and Mercury content of fault gas can reflect the tidal stress and strain of crust. This conclusion gives the theoretical evidence of fault gas which is used in predicting earthquake.