Aiming to solve the problem that the bandwidth is not enough and the VGA monitor is unstable when using video D/A to convert LCD to VGA, the paper presents an implement method of controller that uses FPGA to cooperate with video D/A processor. This method stores the LCD video data that comes from MCU into SRAM for buffering, then read it regularly as the VGA rules need and send to D/A, which converts LCD to VGA, the whole process is controlled by FPGA, and it effectively solves the problem that video output occupies too much bandwidth. this system has already been used in some monitor and display system on the train.