3D contact finite element model of output helical gear pair of wind turbine speed-increase gearbox is built to calculate stress-strain of gear pair under static load. S-N curves of gear pair materials are calculated based on fatigue test constants of materials. After carrying out rain-flow counting for load histories of real load spectrum in 20 working conditions of the gearbox，the relation of load cycles，mean value and amplitude of the load are obtained. Fatigue life of helical gear pair is analyzed with FE-SAFE to study the influence of magnitude of load，residual stress，tooth surface roughness and profile modification to fatigue life of gear pair. The results indicate that there are a few short life points at places where stress concentrates；the life of gear pair extends with the decrease of load or tooth surface roughness；residual tensile stress leads to a reduction of fatigue life while residual pressure stress leads to extension；the life of gear pair extends with proper tooth modification.