Abstract: On the basis of the proposed transformation theory of soft-terrain spectrum, effective spectra under the driving and driven wheels of Changchun - 40 tractor are predicted. According to difference of effective soft-terrain spectra of front and rear wheels of a two-wheel drive tractor, an excitation spectrum matrix of four-wheel input is established when tractor moves on soft-terrain. Correctness of the results measured by the measuring gauge of soft-terrain initial pectrum and the proposed transformation theory is verified through surveyor' s pole measurement method of terrain roughness and dynamic response analysis of the tractor excited by soft-terrrain.