Based on the current situation of stress direction change and its mechanism insufficiently understanding during oilfield development, anisotropic drilling from four cores in Chang-6 oil layer group of a block of Ansai oilfield in Ordos basin, stress direction relative change is measured respectively in dry sample and stratigraphic condition and analyzing the machanisms of effects. Results indicate that there is an obvious difference under the dry sample and stratigraphic condition, which suggests that it has an important meaning to test stress under in-situ condition. Maximum horizontal principal stress direction changes from 4.65° to 8.73° during oilfield development, the average is 7°, and analysis of characteristics of reservoir physical property and mechanism of rock fracture suggests that “natural channel” has an important control function to in-situ stress direction change. Combined with the initial in-situ stress direction, current stress direction of the study area is regarded as about NE58°. The conclusion does make crucial sense to oilfield reconstructive measure such as well network adjustment and artificial fracturing.