A method and device that reduces block, mainly including the steps of: judging whether executing filter operation for current block according to high level differentiation value information of pixel value and determined threshold value; or determining flatness of region according to high level differentiation value information of pixel value and determined threshold value. In the embodiment of the invention, guiding high level differentiation value representing changing degree better in process of filter judging and flatness of region determination improves effectively accuracy of the filter judging so as to reduce block effect of image in decoding process.