The invention discloses a size-reducing decoding method and a size-reducing decoding system. The method comprises the following steps: acquiring a motion vector and a DCT coefficient matrix of a preliminarily decoded macro block by carrying out variable length decoding and inverse quantization of a to-be-decoded macro block; acquiring a second DCT coefficient block and a third DCT coefficient block by carrying out the low-pass filtration of a first DCT coefficient block in the DCT coefficient matrix; acquiring a first reference macro block, a second reference macro block and a third referencemacro block by carrying out inverse DCT transformation of the first, second and third DCT coefficient blocks; according to a required reduced size, calculating the pixel values of pixel points in a reduced-size macro block by means of the pixel values of pixel points in the reduced-size macro block at the mapping points of two of the first, second and third reference macro blocks; and carrying out motion evaluation and motion compensation of the size-reduced macro block by means of a prestored reference image and motion vector acquired through decoding the motion vector of the preliminarily decoded macro block and reducing the decoding result according to the required reduced size.