The aim of this work was to study the auxiliary marker of image location and its selection principle based on print whole image detection system. Bar specimens of four-color ink printing with density gradient were prepared. The density was tested using spectrodensiotometer, followed by scanning and output of RGB value. The influence law of different color auxiliary marker on the RGB response values of CCD image sensor under different density values was studied, and the relevant formula was deduced for describing the significance of image location boundary feature. It was found that yellow, cyan, magenta ink had a good complementary relationship respectively with blue, red, green response of CCD camera, while black ink had a good neutral relationship with these colors. The auxiliary marker response of complementary color channel was quite different from that of the body image, and its positioning boundary significance was more prominent. In conclusion, as a general rule, monochromatic color ink of complementary relationship with the main color should be chosen as the auxiliary marker of image location. When the main color is difficult to distinguish, a general black marker can be considered.