Artificial Intelligence(AI) has achieved unprecedented success in various fields including image/speech recognition which is useful for edge computing. Most of AI systems are implemented on power-hungry devices like GPU, high-end FPGA, or even TPU to process data with high performance. However, these energy budgets are often not affordable to edge computing. Low-end FPGA taking advantage of high energy-efficiency is a desirable platform to meet the requirements of image recognition working on small autonomous vehicles. In this paper, we propose the design methodology and implementation to adapt a neural network system to a low-end FPGA board using HLS description. The whole design consists of algorithm-level downscaling and hardware optimization. The former emphasizes the model downscale by considering accuracy. The latter applies various HLS design techniques to speed-up the application running on the target board. In the case study of tiny YOLO (You Only Look Once) v3, the model running on PYNQ-Z1 presents up to 22× acceleration comparing with the PYNQ ARM CPU. Energy efficiency also achieves 3× better than Xeon E5-2667.