The present invention provides a video picture coding method, a video picture decoding method, a coding device, and a decoding device. The method includes: determining a motion vector difference of an affine picture block; determining motion vector precision of the affine picture block; determining a size of an affine motion compensation picture subblock in the affine picture block according to the motion vector difference, the motion vector precision, and a distance between control points in the affine picture block, where the control points are pixels used to determine the motion vector difference; and performing coding processing on the affine picture block according to the size of the affine motion compensation picture subblock. In this way, a picture subblock of an appropriate size is selected, so that coding complexity can be reduced, and coding efficiency can be improved.