The invention relates to an image day and night scene change method and system. The method comprises the following steps that S100, an image training set is decomposed, and image training blocks are selected; S200, the image training blocks are subjected to dictionary training to obtain dictionary matrixes DR, DG and DB; S300, night image matrixes R, G and B which are preprocessed and the dictionary matrixes DR, DG and DB are used for reconstructing image blocks, and the image blocks are combined to realize image day and night scene change. According to the image day and night scene change method and system, images shot in darkness and images shot under bright light are exchange, and video monitoring can be conducted effectively in darkness, so that the images in darkness are as clear as those at daytime, some regions which do not exist in a sample image are highlighted at the same time, and the image recognition capability in darkness is improved; moreover, the defects that night images in video monitoring are blurred in a public place and the amount of effective information is small can be overcome, and therefore the image day and night scene change method and system have wide application prospect.