The present invention relates to the technical field of video monitoring application, disclosed are a video image quality diagnostic system and method thereof. In the present invention, on the basis of a video monitoring system, the video image quality diagnostic system is incorporated into the video monitoring system as a dedicated diagnostic network, thus effectively reducing the bandwidth consumption of the video monitoring system and the connecting port consumption of a monitoring device. A polling host polls each video monitoring point via software to view a video image, acquires a code stream, and simultaneously transmits the same code stream to a video image quality diagnostic server for diagnosis via the dedicated diagnostic network; the transmission of the code stream does not occupy the bandwidth of the video monitoring network. While polling each video monitoring point to view the video image, the polling host takes a screenshot of the software interface, segments the screenshot image, and then transmits the image to the video image quality diagnostic server for diagnosis via the dedicated diagnosis network, thus further reducing the bandwidth consumption of the dedicated diagnostic network.