An in-arm hydro-pneumatic suspension unit (ISU) has greater load capacity with smaller and lighter size than a normal hydro-pneumatic suspension. The ISU of a tracked vehicle combined with the semi-active control system is used to analyze its dynamic characteristics. A semi-active two-control-loop controller using an on-off skyhook control algorithm is proposed. The outer loop calculates a target spool position that can deliver the required damping force, and the inner loop tracks the required spool position. The integration interfaces among Virtual. Lab, AMESim, and Matlab are studied, and the virtual experimental platform for co-simulation of a mechanical-electrical-hydraulic system is established. The validity of the proposed control algorithm is proved under the platform. The simulation results show that the semi active suspension control system has better performance than the conventional suspension system.