The double closed-loop control system of current source SPWM inverter (CSI) is designed to meet the need of battery pack testing system. The battery pack has tiny inner resistance, back electromotive force and it acted as a comparative load which made the control system hard to design. The control system aimed at the specialty of the battery pack load and put forward a loop-locked control method based on dq coordinate conversion of CSI. Typical I and II control system is adopted to get a better performance. It increases the transform efficiency by SPWM and gets high power factor and high dynamic response quality by dq coordinate conversion. Simulations and tests proved the correctness and feasibility of the control system.