The andesite of the Yixian Formation in Gushan area, Harqin Zuoyi(Left Wing) Mongolian Autonomous County,western Liaoning，overlies on lake facies sediments （the Jingangshan Layer), and is covered disconformitely by the Jiufotang Formation．Both laser ArAr and SHRIMP UPb dating technologies were employed to measure the age of andesite． The results show that it was formed about 126 Ma ago．Compared with the Yixian Formation strata sections in Yixian area， it is found corresponding with the second subcycle of the Yixian Formation volcaniccycle．So，the Jiufotang Formation should form later than 126Ma. Furthemore，the forming age of andesite in Gushan area， the basaltic—andesite of the Yixian Formation underneath the Jianshangou Layer in Sihetun area， the Zhuanchengzi Layer of the Yixian Formation in Yixian area and the Dawangzhangzi layer of the Yixian Formation in Lingyuan area are almost the same each other， indicating that the lake facies sedimentary Layer(the Jingangshan Layer) of the Yixian Formation in Gushan area is earlier than the Jianshangou Layer in Sihetun area，the Zhuanchengzi Layer in Yixian area and the Dawangzhangzi layer in Lingyuan area．