The probe volume of medical endoscope is limited, the use environment is special, and the lighting conditions are limited, so the dark part processing of image is particularly important. This design proposes a fast dark part enhancement algorithm for color image. Firstly, the collected RGB888 format video stream images are channel separated, and then the mean filter of specific window size is carried out for each channel through convolution to extract the regional features. Finally, the average value of each corresponding point of the three channels after filtering is taken and substitute it into the Logistic function of specific parameters. The result is the gain of the pixel point at that position of the frame image, which is applied to the original image. The design takes the Zynq series ARM+FPGA SoC platform launched by Xilnx as the carrier, uses Vivado HLS to develop the AXI-Stream interface video stream processing algorithm, and generates IP to run on the FPGA. After experiments, this algorithm only needs 1.6 milliseconds to process a frame of 400*400@30fps on the Zynq7020 platform, which ensures the real-time output of the video stream. At the same time, the endoscope is inserted into the oral cavity to observe the video stream images before and after the algorithm processing.The algorithm enhances the brightness of the dark part while ensuring the regional contrast, and improves the dark part quality of medical endoscope video stream.