By adding some original colors to CMYK, Hi-Fi printing technology can expand the gamut. The original colors added have substitution relation with CMYK. The substitution relation can simplify the complex Hi-Fi color separation model and improve precision. The R in CMYKRGB 7 colors Hi-Fi model was taken as an example, and gradient-type control trip was designed and printed. The density was measured. The mid-color balance curve was fitted by polynomial regression algorithm to build the alternative equation. The substitution between R and MY was obtained by setting the mid-color replacement rate. 4 groups of different mid-color replacement rate were set and ΔE was measured to determine the proper range of the mid-color replacement rate. The results showed that the accuracy of the separation model is improved by setting the mid-color replacement rate as 0.2.