In view of the traditional P algorithm is easy to be interfered by the road markings, this paper designed a kind of improved P algorithm. It is used to obtain binary image for containing or not containing road crack image. The crack image which does not contain the road marking is the traditional P algorithm, and the crack image containing road marking were thinning of three cases. The experimental results show that, with the traditional P algorithm, the improved algorithm for pavement crack image segmentation and subsequent image processing to produce good results.