Laser powder bed fusion (LPBF) is widely employed in metal additive manufacturing to fabricate components with outstanding mechanical properties and precise dimensions by melting powder layer-by-layer. As an in-line monitoring technique for additive manufacturing (AM), laser ultrasonic testing (LUT) is expected to be effective. During the LPBF process, ultrasonic signals are affected by thickness variations of specimens. This study analyzes the transmission of ultrasonic waves at different thicknesses and the variations in wave types. Realistic AM surface roughness data were incorporated into the simulation model to generate ultrasonic signals at various thicknesses. Subsequently, experimental studies were conducted. The research findings demonstrate that the Lamb wave characteristics are most prominent when the thickness is 0.2 mm. As the thickness increases, there is a gradual attenuation of the Lamb wave dispersion, accompanied by the emergence of surface wave features. The Lamb wave behavior diminishes as the thickness exceeds 3.021 mm, and surface wave, transverse wave, and longitudinal wave become more prominent. The dispersion curves were derived using the f-k method, and the thickness of LPBF Ti-6Al-4 V specimens smaller than 1 mm is precisely inversed based on dispersion curve. The verification experiments demonstrate that the model solution for thickness has a relative error of less than 5 %. Therefore, the proposed method overcomes the constraints of ultrasonic echo thickness measurement that cannot accurately measure thin specimens, while accomplishing non-contact evaluation based on laser-induced ultrasonic.