A three-dimensional heat conduction model of wind turbine blade with defect is established based on equivalent source theory, which solves the problem that one-dimensional heat conduction model cannot effectively predict the three-dimensional heat flow around the anisotropic material defect, and the accurate evaluation of the defect depth of the large-scale wind turbine blade is realized with pulsed infrared thermal imaging technology. The three-dimensional heat conduction equation of the blade with defect is simplified to isotropic problem with linear coordinate transformation, and then the analytical solution of the surface excess temperature in the defect area under the third boundary condition is obtained using separating variable method. Finally, the quantitative relationship among geometry dimensions of the defect, the peak time of excess temperature and the depth of defect is established. The on-site detection of 1. 5 MW wind turbine blade made of GFRP composite material was carried out, which proves the feasibility of the proposed method, and the detection results show that the detection range reaches 7. 8 mm, the detection error is less than 10% , and the detection accuracy is improved by 10% ~ 31. 4% compared with the one-dimensional model method. In addition, when the defect depth exceeds 3 mm, the boundary heat transfer cannot be ignored, otherwise more than 10. 0% detection error will be caused. The method proposed in this paper can provide a reference for the defect detection of other anisotropic materials.