Abstract: Based on acoustic elasticity theory of critical refracted waves(L_CR), the average residual stress at different depth of specimens can be measured by L_CR wave with different excitation frequencies. Piezoelectric ultrasonic transducers have fixed excitation frequency, so it is very inefficient to replace the corresponding frequency piezoelectric transducers to measure stress in different depth. In order to solve this problem, according to the excitation principle of electromagnetic acoustic transducer(EMAT), a design scheme of electromagnetic ultrasonic excitation device that can excite L_CR waves of different frequencies was proposed. The device was applied to detect stress in different depths of specimens and it is conducive to improve the efficiency of stress detection. An electromagnetic ultrasonic critical refraction longitudinal wave excitation device was designed, and the excitation effect of L_CR waves with different frequency was simulated and analyzed. The test pieces of stress varying with depth were prepared, and the stress in different depth was tested based on the electromagnetic ultrasonic L_CR excitation device, which verifies the validity of the method about detecting stress distribution in different depth of metal structure.