Abstract: Bi-layer thin plates are widely used in the construction industry, aerospace, coating technology, etc. The thickness is an important parameter of slab materials, so the characterization of thickness is of great significance. This paper proposed a method for measuring the thickness of each layer of the bi-layer thin plates based on the cut-off frequency of ultrasonic Lamb waves. It was theoretically deduced that cut-off frequency of the bi-layer thin plates is related to the thickness ratio of the two plates. The thickness of each layer was characterized by the total thickness and the thickness ratio of the two plates. The cut-off frequencies of the first-order anti-symmetric mode (A₁) of the bi-layer thin plates (non-aluminum/aluminum) with different thickness ratios were calculated by theoretical formulas, resulting in a curve of the A₁ mode cut-off frequency with the thickness ratio of the two plates, and at the same time the effect of non-aluminum plate's transverse wave velocity on its change was analyzed.