Provided are a method and device for coding an image, a method and device for decoding an image. The method for coding the image includes that: coding mode parameters and parameter groups of one coding block are divided into multiple types of coding mode parameters and parameter groups corresponding to the multiple types of coding mode parameters according to a specified rule respectively; Quantization Parameters (QPs) included in the multiple types of coding mode parameters are determined according to a preset target bit rate; a QP of the coding block is determined according to reconstruction quality for the coding block; a coding mode parameter to be used is selected from the multiple types of coding mode parameters according to the QP of the coding block, a parameter group corresponding to the selected coding mode parameter is set, and a QP difference is calculated; and the coding mode parameter, the parameter group used by the coding block and the QP difference are written into a video bitstream.