In order to study the visual characteristics of the human eye on color-distinguishing threshold levels, an experiment, of which the results provide significant data reference and theoretical basis for improvement of color evaluation methods of printing products, was carried out with 20 observers whose visual characteristics are normal. The observers were provided 5 basic colors that the CIE recommended for color evaluation study, and the constant stimulation psychophysical method was used in the visual experiment. The color patches were printed out with small color threshold variations. The color-distinguishing thresholds and visual characteristics were obtained through the experiment. Results showed that the notable color threshold values of green and red patches were relative larger, indicating larger visual tolerances. It is also shown that the red region in CIELAB color space has the best uniformity to human eyes among the five colors.