Aiming at the difficulty of small target detection in current target detection technology, a small target detection algorithm named improved Bi-directional Single Shot multibox Detector (Bi-SSD) based on Single Shot multibox Detector (SSD) is proposed. This algorithm designed a small object feature improvement module for the shallow features of SSD. In the classification and regression parts of the network, a 6-scale Bi-directional Feature Pyramid Network (BiFPN) is designed as classification and regression sub-network according to multi-scale feature fusion method and BiFPN structure. Experimental results show that Bi-SSD has better detection performance than the original SSD on PASCAL VOC and MS COCO object detection datasets. On VOC2007+2012, Bi-SSD achieves 78.47% mAP, which is an increase of 1.34% compared to the original SSD algorithm. On COCO2017, Bi-SSD achieves 26.4% mAP, which was an increase of 2.4% compared to the original SSD algorithm.