The invention relates to a method of measuring the imaging quality of a photoetching machine, and includes the following steps: the DFOCAL-mark images evenly distributed on a mask are exposed onto a silicon chip to form n exposure sites under n defocusing amount of the photoetching machine. An aligning system is utilized to conduct an equally-spaced sample measurement to the n exposure sites to obtain n/m on-site aligning position for each mark. The deviation of the horizontal position of the mark imaging and the variation law of the defocusing amount of the silicon chip are utilized to calculate the approximate position of the optimal focus for each DFOCAL marking point on the exposure site. The corresponding aligning position of the DFOCAL marking point on the n/m sites closest to the approximate position of the optimal focus for each DFOCAL marking point on the exposure site is measured. The optimal focus position for each DFOCAL marking point is figured out precisely. The axial image quality parameter is calculated. The invention can conduct a highly accurate, extensive and fast measurement to the axial image quality parameter for the imaging quality of a photoetching machine by utilizing DFOCAL marks.