The spectrum matrix of six-tire input, comprised of the two-axle and four-wheel of the tractor and the one-axle and two wheel of the seeder, were deduced. The obtained matrix can be applied to research the vibrating frequency domain characteristics of the seeder which is caused by the farm surface roughness. And by applying the R-W method, a dynamics model with 22 freedoms of the seeder was set up. Then the motion and dynamics was analyzed based on the derived dynamics equation. Furthermore, with the method of multiple-step Geal estimation and correction, the time domain was solved, and with the method of equivalent method of statistical linearization, the frequency domain was solved. Comparative analysis shows that the dynamics model is right. 