This paper proposes a novel decoupling vibration control method for the semi-active electrically interconnected suspension (EIS) and develops a mechanical hardware-in-the-loop (mHIL) test platform to verify the effectiveness of the method. Firstly, according to the decoupling characteristic of the interconnected electrical network, the half-car model is divided into vertical and roll ones, and their frequency response could show the capability of EIS on vibration control. Then, two H∞ state feedback controllers are designed based on the decoupled model to acquire the ideal force and torque for vertical and roll vibration control; the adjustment logics of the resistance control unit are proposed to track the ideal force and torque. Finally, the mHIL experiment platform is designed to validate the decoupling vibration control performance of the EIS system. In the experiment, the EIS can significantly improve the vehicle suspension's overall performance under sinusoidal and bump road excitations compared with that of the passive suspension. On the C-level random road, the test results show that the EIS can reduce the root-mean-square vertical and roll angular accelerations of the sprung mass by 22.90% and 21.75%, respectively, which significant improvement the ride comfort of the vehicle.