In order to explore the wear characteristics and mechanism of large gear transmission under cyclic alternating loads and reduce the failure rate of the gear transmission system under the influence of unstable load conditions, the wear characteristics and operation state of the gear under different loads were analyzed. The wear mechanism of gear was studied from four aspects of wear amount, wear profile analysis of tooth surface, oil analysis and abrasive particle analysis. By using the force closure and spring vibration subsystem device, constant and alternating loads were applied; particle counter, analytical ferrospectrometer and scanning electron microscope were used to observe the oil sample abrasive particles and damage morphology of gear tooth surface to reveal the evolution mechanism of gear wear under different loads. The results show that the wear degree and wear mechanism of the tooth surface at different positions are different, and the wear at the dividing circle is light, mainly is contact fatigue wear; the wear at the tooth tip and the tooth root is more serious,mostly abrasive wear and abhesive wear, and the wear phenomenon at the tooth root is the most serious; the impact and vibration of the tooth surface under alternating load is more serious than that under constant load, which causes plastic flow and large area spalling of the tooth surface material.