The safety and reliability of electric vehicle depend on the performance of vehicular energy storage system. The test bench composed by DC motor and flywheel was proposed to test the performance of energy storage system. This paper mainly focuses on an improved test bench. The tracking precision of speed control system was affected by the improved test bench. The model and current problems of the test bench were presented. The structure improvement method of the test bench was improved according to the theory of multi-shaft transmission system. The improved test bench was modeled by using state-space equation method. The efficiency and ratio control of continuously variable transmission (CVT) were analyzed. The dSPACE-based hardware-in-the loop (HIL) test bench was built and improved in the lab. Experimental results showed that the tracking precision of speed control system was improved with new test bench. It provides technology support for the performance testing of hybrid energy storage system.