Abstract: According to the tectonic and sedimentary characteristics of Xuanhan-Daxian area,northeastern Sichuan basin,aimed at the existing geological traits of every set of erosion strata,the authors have restored the strata erosion thickness of the area in the key structure transform period by use of acoustic transit time,Ro-H,fluctuate analysis and thickness of nearby layer contrast,and erected the fluctuate equations of sediment and erosion section of 12 typical wells in the study area.The results indicated that the erosion thickness of the earth's surface(from Jurassic) is 700~2300 m which is shaped since the Yanshanian.The erosion thickness at the top of Xujiahe Formation,Upper Triassic,is 80~150 m,which was formed at later Indo-Chinese epoch.The erosion thickness at the top of Leikoupo Formation,middle Triassic,is 240~450 m which was formed at early Indo-Chinese epoch.