Abstract: This paper offers some approach to determine the principle strain axis of soil layer in situ, describing the basic theory, method, experiment and field application. As mentioned above, the following conclusions is written in this paper.1. It is entirely possible to determine the principle strain of soil layer in situ with the method introduced in this paper.2. The data of determination of the principle strain axis of soil layer in situ are useful for the seismogeology and engineering geology.