The ink mixing experiment shows that the hue angle in the CIEL*a*b* color space is a monotonic function with the quantity of ink. In this paper, the exponential fitting method is used to establish the relationship curve between the hue angle of spot color ink and the amount of primary color ink, and the color matching of two primary color spot color inks is obtained. This method can obtain the proportion of primary color ink more accurately and quickly, and complete the spot color ink mixing. In addition, this method also has certain theoretical significance and practical value for the color matching of spot color inks with three primary colors.