The invention provides a sub-camera video capture device and a method thereof. According to the invention, a scene optical image signal acquired by a camera is divided into at least two paths by a light-splitting unit; at least one path of the optical image signal is directly converted into a real electronic image signal in a photoelectric way; a filtering element is simultaneously used for filtering the rest paths of the optical image signal; then, the optical image signal processed by filtering is converted into a filtering electronic image signal in a photoelectric way; a filtering suspected flame image part is detected from image frame of the filtering electronic image signal; and then, based on the filtering suspected flame image part, a real suspected flame image part is confirmed from the real electronic image signal.