Based on the linear-elastic fracture theory, the fracture damage of gear structure is studied. By using the professional fracture analysis software FRANC3D, a three-dimensional propagation analysis is investigated for tooth root crack in involute gear, and also solving the three types of stress intensity factor(SIF) in tooth root semicircle crack tip, its changing rule and the 3D stress intensity factor formula are obtained. On this basis, a simulation analysis and study is conducted on propagation route of 3D tooth root crack, and the correctness of the analysis results is verified by comparing with the experimental results. Finally according to the mutation of maximum stress intensity factor in the crack tip，the fracture damage of gear teeth is predicated, and its working life is forecasted. By analyzing the different crack size, it is concluded that crack propagation velocity is very sensitive to crack initial length; initially, crack propagation slowly grow, with the increasing of crack length, the crack propagation will rapidly accelerate. The cycle life of fatigue crack in involute gear is about 1.85E5 times. All these findings have important significance for life estimation and failure detection of gear.