In order to solve the problems of low detection efficiency and poor detection precision caused by various surface defects and numerous small defects of section steel， a detection algorithm for surface defects of section steel， namely Steel-YOLOv3， was proposed on the basis of the deformable convolution and multi-scale dense feature pyramid. Firstly， the deformable convolution was used to replace the convolutional layers of part of the residual units in Darknet53 network， which strengthened the feature learning ability of feature extraction network for multi-type defects on the surface of section steel. Secondly， a multi-scale dense feature pyramid module was designed， which means that a shallower prediction scale was added to the 3 prediction scales of the original YOLOv3 algorithm and the multi-scale feature maps were connected across layers， thereby enhancing the ability to characterize dense small defects. Finally， according to the defect size distribution characteristics of section steel， the K-means dimension clustering method was used to optimize the scales of anchor boxes， and the anchor boxes were evenly distributed to 4 corresponding prediction scales. Experimental results show that Steel-YOLOv3 algorithm has a detection mean Average Precision （mAP） of 89.24%， which is improved by 3.51%， 26.46%， 12.63% and 5.71% compared with those of Faster Region-based Convolutional Neural Network （Faster R-CNN）， Single Shot multibox Detector （SSD）， YOLOv3 and YOLOv5 algorithms respectively. And the detection rate of tiny spalling defects is significantly improved by the proposed algorithm. Moreover， the proposed algorithm can detect 25.62 images per second， which means the requirement of real-time detection can be met and the algorithm can be applied to the online detection for the surface defects of section steel.