The parametric model of 4UX—550 potato harvester and suspension device of Dongfanghong—200P tractor were established with Pro/E software, and the assembly of suspension device of tractor and potato harvester was accomplished. The assembly model was imported into ADAMS. The hydraulic lifting loop system model was established. Then, the coupling simulation of mechanical system and hydraulic system were advanced. The results showed that when hydraulic pressure drived suspension unit, top hanging point appearing the biggest force evenly delayed 0.080s than that of the mechanical force driving, and lower hanging point delayed 0.088s under lifting conditions; top hanging point delayed 0.056s and lower hanging point delayed 0.060s under dropping conditions. When top and lower suspension points at the same force under the drive by hydraulic pressure and mechanical, hydraulic cylinder buffer force was in the ranges of 5.09~15.39N.