Screen line number has a great impact on digital printing quality of space non-uniformity. The quality attributes of space non-uniformity, testing equipment, algorithm of quality attribute, and testing process were introduced. The test blocks's granularity, mottle, blank, and noise power spectrum specified in the ISO 13660 were obtained. The situations of the same screen line number with different dot area rate and the different screen line number with the same dot area rate were analyzed for the causes of data formation. The test result was compared with subjective judgments. The result showed that increasing screen line number can improve the digital printing quality of space non -uniformity.