The invention relates to the technical field of operation and maintenance of wind turbines, and is also applicable to non-destructive measurement of a depth of a defect of other resin-based composite materials. It is aimed at the problem that the depth of the defect cannot be determined by an intuitive infrared thermal image in the measurement of a depth of a defect of a large wind turbine blade. This method not only ensures accuracy of the measurement, but can also be widely applied. The method comprises the following steps: S1: continuously heating a surface of a wind turbine blade with an irradiation heat source; S2: collecting and storing a real-time heat map sequence of a surface of the blade with an NEC R300 infrared thermal imaging camera; S3: extracting a surface temperature rise curve at a defect location and organizing the results into a temperature rise curve family of the layers in a depth direction at the defect location; S4: extracting a surface temperature rise curve at a non-defect location, and calculating the similarity between the two temperature rise curve families; S5-S7: obtaining a reference depth value of the defect; and S8: determining whether the depth value is in a characteristic interval.