Abstract: A new method has been introduced,based on theoretical analysis and actual data analysis.By analysizing percentage of mudstone in the hydraulic fracture profile and statistical trend that ISIP changes among selected wells,minimum principal stress inside buried mudstone strata(layers of Chang 8 at 2 100 m,Xifeng oil field and He 8 at 3 000 m,Jinbian gas field in Ordos Basin,China,are cited respectively) can be obtained.Final results indicated that the new way is more appropriate.