In this paper, we have computed the data of hourly values and daily values of radon content by means of the optimal spectrum-an alysis method of tidal wave. The results show that there exist semidiurnal and daily wave components in hourly value sequence of radon content measured at Wuquanshan spring of Lanzhou, which are consistent with the theoretical earth tide; there exist semimonthly and monthly wave components in daily value sequences of radon content measured at Liaocheng drill of Shandong Province, Zhangdaokou drill of Tianjin City, Pingliang spring of Gansu Province, which are consistent with the theoretical earth tide. It has proved that drills and springs having good environmental conditions and the data having good quality, the change of radon content can reflect the change of tidal stress of earth crust.