In this paper, an evaluation method for coherent jamming of synthetic aperture radar based on blur length is proposed. Firstly, a synthetic aperture radar jamming image with blurred effect is generated based on the corresponding template of coherent jamming modulation, by analyzing the error of influence of frequency modulation gradient. Then, the Sobel operator is used to extract the target edge and calculate the average edge width. Considering the influence of the internal pixels of the target on the contour edge in the actual SAR imaging process, the evaluation index of blurred edge width ratio designed. Finally, the index is compared with the classical interference evaluation index based on the simulation images. The experimental results show that the proposed index not only has consistency in the measurement of the classical interference evaluation index, but also has better evaluation result on the interference image blur of point target and small target.