We developed a mechanical power circulation test rig for a wind turbine gearbox with a power rating of 5.8 MW or less. The test rig consists of an electric motor, two auxiliary gearboxes, a torque-applying device, lubrication systems, cooling systems, and control systems. The torque generating device consists of a planetary gearbox and a hydraulic control system and is used to apply the desired torque to the test gearbox. The hydraulic control system applies the torque on the ring gear of the planetary gearbox. The gears and bearings of the two auxiliary gearboxes and planetary gearboxes met the design criteria for a safety factor of over 1.2 and a bearing life of 30,000 h. In addition, the master and slave gearboxes were connected to the test rig to verify whether the torque-applying device had applied variable torque in real-time during the test. The device was only able to induce a variable torque of up to 45.2 kN-m due to the limitation of the rated torque of the master and slave gearboxes. The test rig can test not only efficiency, vibration, and noise but also durability and overloading.