Abstract: Abstract After a certain megawatt level wind turbine was connected to the grid for 3 years, the 4th stage center wheel of the yaw drive gearbox experienced a shaft breakage accident. Using macroscopic observation, safety analysis, chemical composition analysis, mechanical property testing, metallographic examination, scanning electron microscopy and energy spectrum analysis methods, combined with finite simulation analysis, the causes of the center wheel fracture were analyzed. The results show that there were multiple non-metallic inclusions at the fracture surface of the central wheel, led to a decrease in the ultimate strength of the material. The design size of the shoulder transition zone was too small, and there were multiple rough processing knife marks remaining on the shoulder surface, which made the shoulder a dangerous section and generated stress concentration, caused the material to undergo torsional fatigue cracking. The cracks continued to propagate until the strength of the residual material was insufficient, ultimately led to a broken shaft accident in the center wheel.