Car is a complicated dynamic system. It's hard for people to establish an accurate model. The process must be predigested and simplified under the certain condition. The dynamic idiosyncrasy of car has been studied. Based on the mathematic model of eight degree of freedom, car's mass of center acceleration, relatively dynamic load, dynamic flexibility and root mean square value of acceleration with energy method have been concluded. Simulating procedure with Matlab software was also programmed, and some calculating about the car was done. Compared with the experimental results, the simulating outcome met the need of engineering, which meant the mathematic model and deducing were correct.