Dynamic modeling of gear system is an important tool to study the mechanical characteristics and behavior law of gear system in the process of motion. In the theoretical modeling of gear system dynamic, the most commonly used method is the lumped parameter method. Aiming at some shortcomings of the lumped parameter method, a contact dynamic method for spur gear transmission is proposed. This paper systematically compares the similarities and differences between the proposed method and the traditional lumped parameter method. Firstly, according to Newton's second law, the dynamic differential equation of spur gear is constructed, which adopts the same method. Secondly, in order to accurately determine the accurate position of spur gear system in the process of motion, the contact method puts forward a mathematical contact determination method for involute tooth profile, which is characterized by accurately calculating the meshing point position by solving the equation, and the lumped parameter law does not have this step. The contact model is then constructed by the relevant theory of contact mechanics, and the solution is circulated. Although the theory adopted in this step is the same, the result of the proposed method based on the second step is more accurate. Finally, by comparing and analyzing the proposed method, the traditional lumped parameter method and the experimental method, the accuracy of the proposed method compared with the lumped parameter method is verified. Based on the proposed contact dynamic model and the lumped parameter model, the dynamic transmission error of spur gear system under different speeds and loads is studied. The proposed model and research results lay a foundation for the research fields of gear system design, analysis, optimization and fault diagnosis.