In the light of computation divergence causing by zero quality existing of the finite element dynamic model of a plow system，symbiotic time-varying dynamical equations of longitudinal vibration and contact-impact vibration are established based on the quality substitution method considering the factors of plow head movement, the clearance of plow head and sliding framework, pretension force of plow chain and polygon effect of sprocket. The test uses the planning load of bit as excitation. Time-varying dynamical equations are solved with the numerical analysis method. Dynamic characteristics of plow system under different depths are analyzed，and compared with the calculation results of the dynamic model before improvement and experimental results. The results show that the problem of computation divergence are avoided by using the suggested method, and the computational accuracy of the dynamical model are improved. The research results provide theoretical basis for working stability, fatigue life of key parts analysis of plow system.