not-yet-known not-yet-known not-yet-known unknown ：Fatigue damage is the primary failure mode of automotive gears. This study explored the influence of surface strengthening techniques on the transmission performance and contact fatigue life of helical gears in automatic transmission, highlighting the practical value to improve contact performance of gears. Employing 20MnCrS5 as the base material, manganese phosphate conversion coating and composite micro-shot peening were applied respectively to investigate the strengthening mechanism through surface morphology, friction factor, residual stress, and microhardness. Experiments were conducted to analyze transmission characteristics such as surface roughness, efficiency, vibration attributes in running-in stage, the service life and failure mechanism of surface-strengthened gears. The findings indicated that both manganese phosphate coating and composite micro-shot peening can noteworthy heighten the fatigue life of helical gears.