This paper presents an FPGA accelerator for processing SSD object detection. The number of computations can be reduced by the proposed multi-scale spatial attention mechanism. To enhance the efficiency for hardware implementation of the CNN, we propose dynamic quantization on hardware and autonomous memory access control. The developed prototype based on SSD with multi-scale spatial attention mechanism implemented on XCZU7EV exhibits an object detection accuracy of 79.3% mean average precision on the PASCAL VOC dataset. The proposed design achieves high digital signal processor efficiency of 94% and good operation speed of 77.7 msec.