Abstract: In view of the present state of welding flaw X-ray test method and open problem, an improved watershed algorithm is proposed. In consideration of the structure information of image, the valley-bottom value produced by noise is very small. However, the minimum valley-bottom of each area well have a very big dynamic value corresponding to real area, which is close to the valley-bottom dynamic value when there is no noise. Hence, the valley-bottom produced by noise can be flitered, thus effectively restraining the over-segmentation, provided that a threshold is simply given based on the dynamic combination rule. Experimental results show that the algorithm can quickly and accurately obtain the segmentation result of flaw image. Futhermore, it has higher ability in resisting noise.