Tristimulus weights single-constant mathematical model of computer color matching of spot-color printing ink used in offset printing was established based on Kubelka-Munk theory. The new mathematical model and Allen’s conventional tristimulus mathematical model were programmed using Matlab software. Computer color matching experiments showed that the tristimulus weights single-constant mathematical model is applicable to offset printing spot-color ink; the number of iterative correction is reduced; color difference is smaller, and metamerism degree is reduced compared with the conventional tristimulus mathematical model. It was concluded that the new mathematical model has higher color matching accuracy.