In the power add and subtract system of high speed dynamometer, unbalanced dynamic load would be produced during absorption and testing of the power generated by the engine. Its strength and stiffness must meet the design requirements in order to absorbing vibration energy. According to the relevant technical requirements of high speed dynamometer, structural design of the mounting rack was completed, its calculation model was built based on ANSYS and the finite element analysis was done, ensuing the design of the mounting rack meet the requirements of stiffness and the lowest natural frequency. In addition, the inherent characteristics of the mounting rack in the load state of the equipment were obtained by test. The maximum error was less than 5%, comparing with the numerical results, which shaw the effectiveness of the numerical calculation method. Based on this, the dynamic response characteristics were studied systematically to ensure the safe operation of the engine unit.