Surface defect inspection for underwater structures is important. However, the inspection technologies based on passive vision cannot meet accuracy requirements. In this paper, we propose a two-stage method based on structured light images for defect detection. In the first stage, light stripes are extracted based on the analysis of hue, saturation, value (HSV) space and gray space. Then a hole-filling method is applied to ensure stripe integrity. In the second stage, depth information for all light stripes is calculated to synthesize a depth map, which is segmented for defect localization and measurement. Experimental results have verified the feasibility and effectiveness of our method.