Aiming at the problems of selective laser melting (SLM) processing of GH3536 alloy, the difference in acoustic impedance between the unfused small defect and the intact area is small, and the ultrasonic echo defect defect is not obvious,this paper presents a signal processing method based on variational modal decomposition. Ultrasonic testing is performed on different micro-sized defect specimens, and the db7 wavelet function is used to decompose the signal into three layers. Then, the minimum and minimum （Minimaxi）rules are used as thresholds to perform hard threshold noise reduction and reconstruct the signal to extract defect echoes in the signal mutation points to identify defects. Results show that the method has obvious noise reduction effect, can detect the smallest detectable defect size of 0.15 mm and significantly improves the sensitivity of defect detection.