The invention discloses a tri-axial testing device for testing the water content distribution of a soil sample. The device comprises a constant current alternating current source, a dynamic voltage tester, a computer and a plurality of groups of sheet electrodes, wherein the sheet electrodes correspond to the layers of the columnar tri-axial soil sample; each group comprises four sheet electrodes which are sequentially adhered to the corresponding layer of the soil sample and a tri-axial membrane at an interval of 45 degrees along the circumferential direction of the corresponding layer of the soil sample; the farthest two sheet electrodes and the middle two sheet electrodes in each group are alternately connected with the two electrodes of the constant current alternating current source and the two input ends of the dynamic voltage tester; the output end of the dynamic voltage tester is connected with the computer. According to the device, the van der Pauw principle is used for dynamically measuring the water content of the soil sample in real time; due to the adoption of the flexible sheet electrodes, the influence on the mechanical properties of the soil sample is avoided, and the testing accuracy of the water content of the soil sample is greatly improved.