Due to the particularity of the aviation industry, the specific mechanical properties of the aircraft parts provided by some external suppliers cannot be obtained, which makes it difficult to calibrate the accuracy of the strain and load monitoring results in the ground and flight tests. In order to meet the requirement of measuring the elastic modulus of solid structures with unknown material properties, this paper presents a measurement method based on the principle of late echo formation in ultrasonic testing. The transverse wave velocity of materials is measured by using the commonly used vertical ultrasonic probe, and the elastic modulus of materials is calculated according to the inherent relationship among the elastic modulus, Poisson’s ratio, density, longitudinal wave velocity and transverse wave velocity. This is a general method to measure the elastic modulus of solid structures with unknown material properties without additional test blocks, transverse wave probes and complex test equipments. Experimental results show that this method can measure the elastic modulus accurately.