In terms of the problem that the signals obtained by laser ultrasonic testing technology applied to metal additive parts are complex, multimodal and with low signal-to-noise ratio, the laser ultrasonic signals are obtained for time-frequency analysis to explore their frequency domain separability. The variational mode decomposition algorithm is used to separate and extract the best surface wave mode according to the frequency domain characteristics. On this basis, a technique based on B-scan of laser ultrasonic signal combined with variational mode decomposition to extract surface echo eigenvalue is proposed for the quantitative detection of the surface crack length of metal additive parts. In view of the problem that there is a large error in obtaining crack length information by directly observing the B-scan image, the change of the peak to peak value of the reflected echo of the surface wave mode extracted by the variational mode decomposition is analyzed to detect crack. The scanning position-peak to peak value diagram is drawn, and the start and end positions of the crack can be accurately obtained according to the diagram. The relative error of the test results does not exceed 8%. Compared with directly obtaining the crack length information of the original B-scan image, the detection accuracy is improved. This method is feasible in feature extraction and quantitative detection of laser ultrasonic signals of metal additive parts.