Abstract: Ceremic plate extractor is used to test not only the unsaturated soil drying curve,but also the wetting curve, which has important meaning to the study of the physical relation ship between soil and water. To improve the present ceremic plate extractor, I recommend using electronic pressure gauge to improve the accuracy of the Suction control, using electronic balance and setting evaporation-control group to realize the accurate measurement of moisture in soil sample variation, establishing a data acquisition system to avoid the error accumulate caused by testers in recording process. Mu Us sand was tested by using this device, the test data are fitted by using the Van Genuchen, Gardnera, Fredlund and Xing model. Conclusion them have better fitting effects. Determine the boundary point of the unsaturated soil wetting and drying curve of steep drop section through the geometric mapping method.