Abstract: In order to solve the problems of ignoring anisotropy of soils and single loading direction in the traditional in-situ soil deformation testing, a design concept of anisotropic deformation in-situ testing system for loess is proposed. The entire testing system consists of support system, cutting system and detection system, similar to the in-situ anisotropic horizontal load tests in boreholes. A prototype is successfully developed. The prototype can be fixed at any depth in the borehole by the support system, a plane can be cut out by the cutting system, and the borehole can be squeezed by the array of squeezing plates, and the pressure and displacement data can be recorded by the data acquisition devices, thus obtaining the stress-displacement relationship curve. The reliability of the testing system is verified based on the field tests. The test results show that the modulus of deformation varies inversely with the water content and positively with the soil cohesion and peak cone resistance, indicating that the anisotropy of loess is related to the water content and peak cone resistance of the soils. Within a certain range, the difference in the deformation modulus between the soils at the same depth in different directions decreases with the increase of the water content. With the increase of the peak cone resistance, the difference in the deformation modulus between the soils at the same depth in different directions also increases. The reliability of the test results obtained by the testing system is thus verified.