With the development of helicopter technology, there is an urgent need for quality inspection and research on a new type of helicopter blade iron-clad structure. The outer layer of the iron-clad structure is stainless steel, the middle layer is rubber and heating elements and the inner layer is glass fiber. The four types of typical structures that need to be classified and identified are the upper layer debonding defect, the heating element, the lower layer debonding defect, no defect and no heating element, but this kind of iron-clad structure has more layers, thinner layer thickness, more categories to be identified than the common multilayer structures, and the ultrasonic waves will have serious aliasing in this structure. Therefore, the propagation law of ultrasonic waves in the thin-walled multi-layer structure is studied, and a new ultrasonic reflection spectrum model of the iron-clad structure is established. The tested stainless, rubber, sound speed of the glass fiber, acoustic impedance, and frequency-attenuation coefficient curve, frequency spectrum of the 2.25 MHz and 5 MHz probes is brought into the model for simulation. The simulation results show that the 2.25 MHz probe can identify four structures of the iron-clad iron, and the 5 MHz probe can identify the other three structures except for the lower layer defects. The actual test results verify the simulation results of the new model.