Abstract: The fatigue crack growth tests were carried out on the single-edge notched specimens of 2A12-T4 aluminum alloy with different thicknesses. The crack front morphology during stable crack growth was observed and fitted by the elliptic equation, and the fitting parameters of elliptic equation were obtained. The mathematical expression between the relative tunnel depth of crack front and the specimen thickness was established. The critical thickness of the crack front morphology transition during growth was predicted and verified by experiments. The results show that with the increase of the specimen thickness, the difference of crack length between the surface layer and the inner layer gradually decreased. The fatigue crack front morphology during stable crack growth changed gradually from "tunnel" shape to a straight line perpendicular to the surface. The relative tunnel depth of crack front decreased with the increase of the specimen thickness, and tended to 0 with the thickness of 31.5 mm. The test results verified the accuracy of the predicted results.