The invention discloses a lossless video compression method and system for a digital KVM switcher having low delay and low complexity. The lossless video compression method comprises the following steps of: (1), writing frame-level coding information of a screen video image in a code stream; (2), sequentially inputting RGB data of pixels to be compressed in the screen video image; (3), calculating the prediction mode of the current pixels according to the spatio-temporal correlation; (4), calculating the coding mode of the current pixels according to the prediction mode of the current pixels; (5), writing a compression bit in the code stream according to the coding mode of the current pixels; (6), repeating the steps (2)-(5), such that coding of all the pixels in the screen video image of the current frame is completed; and (7), performing byte alignment filling of the code stream of the current frame, such that coding of the screen video image of the current frame is completed. The method disclosed by the invention is applied to compression of the screen video image of the digital KVM switcher; the compression processing delay and the code stream data volume of the screen video image can be effectively reduced under the limited calculation complexity; and simultaneously, lossless video quality can be kept.