In order to solve the problems of poor separation effect, low potato-exposing rate, high damaged and bruising rate of potato mechanized harvesting under the clay in southwest mountainous area, a multi-stage separation buffer potato harvester was designed by adopting the methods of multi-stage separation vibration, multiple buffering and low-level side output. On the basis of expounding the overall structure of the machine, the design features of the main components and the operational principle, the structural parameters of the key components, such as the excavating mechanism, the multi-stage separation buffer device and the active vibration device, were determined through theoretical calculations. The motion model of potato and soil in throwing separation stage was established to obtain the technical characteristics of micro-wave potato soil separation. The potato sliding model of sieve surface and the theoretical analysis of potato tuber motion characteristics was carried out, and the motion law of potato tube downstream and back flow was obtained. The collision characteristics of soil blocks during transportation and separation were analyzed to determine the influencing factors of separation and crushing effect. In order to obtain the optimal speed parameters of each stage of separation screen in field test, taking impurity content and soil coverage as the test index, the quadratic rotary orthogonal combination experiment design method was adopted in no-load test. The real-time distribution and motion law of soil on separating sieve were obtained by using high-speed photography and three-axis attitude sensor. And the results showed that when the linear speed of the first-stage separating sieve and the second-stage separating sieve were 1.42m/s and 2.2m/s, and the linear speed of side output was 1m/s, the soil coverage was 69.11% and the impurity content was 2.56%. Under the condition of the optimal speed parameters, taking the potato-exposing rate, bruising and damaged rate as the test index, the orthogonal harvest experiment was carried out in the field. The results showed that when the working parameters were as follows: working speed was 1.05m/s, digging depth was 180mm, vibration strength was grade Ⅱ and inclination angle on the sieve surface was 22°, the obvious potato rate was 99.1%, the bruising rate was 1.41%, and the damaged rate was 1.32%, which were superior to national industry standards.