In order to decrease the bit-rate of video encoding while keeping quality of video, considering the parallel feature of FPGA and block processing feature of HEVC, a block based Gaussian background model-ROI extraction mapping algorithm is proposed and implemented for HEVC video encoder. Firstly, background frame is set by this method, and current frame is classified as foreground and background by SAD rule, then mapping them into coding tree unit(CTU). Secondly quantization parameter is adjusted according to distortion-rate automatically, well followed by the HEVC encoding procedure. The experimental results show that the speed of this algorithm on FPGA has reached the speed at 22 fps@1 080 p, and the bit-rate is saved about 10% with stable video quality.