Abstract: Based on many assumptions,the test error of the maximum principal stress of borehole cross-sections by the classical hydraulic fracturing may be great.Hydraulic tests on preexisting fractures make full use of the merit that normal stress of fractures can be determined by reopening tests on fractures,thus improving the test accuracy,and it can make large scale 3D stress measurements.But if the distance among fractures is large,the hydraulic tests on preexisting fractures should consider stress gradient.The theory and test procedures of hydraulic tests on preexisting fractures are introduced.Based on a large number of research findings of regularities of stress distribution,the simplified stress gradient for general engineering is acquried,and special calculating formulas are derived.From the engineering applications,it is found that the test results by the hydraulic tests on preexisting fractures considering stress gradient can more clearly reflect the change of stress field,and there exists certain difference between the test results by the hydraulic tests on preexisting fractures without considering stress gradient and those considering stress gradient. 更多