As a main bearing part of in-service aircraft, horizontal stabilizer shaft has a slender cavity with variable thickness section. Stress concentration often occurs at the variable thickness section, which results in fatigue crack and thus lead to fatigue fracture. In this paper, an on-line monitoring method of crack depth at variable thickness section based on active Lamb wave is studied.First, the real damage produced by wire cutting is monitored by active Lamb wave, and the response signals are extracted by Shannon continuous complex wavelet transform to eliminate noise. Second, the response signals are analyzed by 4 damage indices emphatically to characterize the rule of crack depth growth. The result shows that the cross-correlation damage index of A0 mode can effectively characterize the rule of crack depth growth best. Finally, the cross-correlation damage factor of A0 mode is used to monitor crack initiation and crack size quantification, which lays the foundation for the on-line monitoring of the horizontal stabilizer shaft. This research has an important application value in realizing the on-line monitoring of the crack in horizontal stabilizer shaft and guiding the service time prolong as well as maintenance of in-service aircraft.