In this paper, a mechanism which can adjust inertia significantly is presented. Such mechanism contains a flywheel, a group of clutches, a direction converter, and a magnetorheological hydraulic rotary actuator. According to the physical definition, inertia is the resistance of object changing in its state of motion, which always connects to the mass of the object. Inertia is also known as an inherent property of an object. Conventionally, among impedance factors of the mechanical system, inertia seems to be unchangeable, not like damping factor or elasticity. However, changing inertia might bring different behavior to mechanical networks. In this study, instead of changing the mass of the object, by controlling the flywheel energy collected from the motor pump to release to the object, the resistance of actuator motion can be adjusted. We call this the pseudo-inertia adjustable mechanism (pIAM). The clutches are used for controlling the engaging and disengaging of the flywheel, energy releasing direction and magnitude. The mechanism design, mathematical model, and experimental results are included in this paper. According to the results, the machine is able to release the maximum torque about 7 Nm and the maximum speed about 800 degrees/s with 1000 rpm of the flywheel speed, which reveals the feasibility of this mechanism.