The traditional Automobile-inertia -brake-testing-system adopts the Mechanical Inertia pan to imitate automobile the Moving Inertia. This kind of system volume is bigger than new one, more difficult in inertia adjusting, and lower in automation degree. Aiming at the problems, this paper puts forward the innovative design of Automobile-inertia-brake-testing-system based on electrical inertia. The mathematic model of the Mechanical Inertia is analyzed. To use the algorithm to control motor, the authors make the output inertia generated by the controlled motor can imitate the Mechanical Inertia, then apply electrical inertia to replace Mechanical Inertia .The simulation result shows that the design idea is correct and feasible, stronger in system rubustness, lower in cost, more accurate in control quality, and higher in automation level.