Abstract: The stress state is the basis for evaluating the deformation and strength of soil. The maximum principal stress and its direction are the important basis for determining the generation and development process of sliding surface of slopes, foundation pits and other structures. To study the real three-dimensional stress state of a point in the soil, based on the calculation relationship between the normal stress and the stress state and considering the test errors caused by the rotation of the device in the test process, a test device named three-dimensional true earth pressure sensor is designed to measure the normal stress in 6 directions and 3 angles around the axis in soil. The three-dimensional true earth pressure sensor is composed of a high-strength polyhedron base, 6 micro earth pressure cells, a micro pore water pressure cell, and a micro inclination sensor. It can determine the total stress and effective stress in 6 directions, and calculate the total stress state and effective stress state of soil. The three-dimensional true earth pressure sensor is used in the one-dimensional loading-unloading cycle of a fly ash, and the test results are compared with the theoretical values calculated by the Boussinesq method. Meanwhile, the 3 principal stresses and their directions, the Bishop constant (b), and the Lode's angle (θ) are calculated according to the obtained three-dimensional stress state, and the relationship between the vertical stress and the vertical strain in each loading process is analyzed. The results show that the rotation angles caused in the embedment and test processes will cause test errors, and these errors will increase with the increase of the test values. The direction of the maximum principal stress does not coincide with the loading direction in the one-dimensional loading-unloading process. With the increase of loading- unloading cycle times, the value and direction of the principal stress will gradually stabilize in a range, and the stress-strain relationship of soil also tends to be gentle. The mechanical parameters determined by the three-dimensional true earth pressure sensor can be consistent with the theory of soil mechanics, and the proposed device has a technical basis for the study of soil strength.