Aiming at the problem that the accuracy of the traditional Kubelka-Munk(K-M) theory spectral prediction model is not ideal when performing offset printing spot color ink spectrum prediction, a color spectrum prediction method for offset printing spot color ink is proposed by constructing a base ink database based on piecewise linear interpolation method, combined with modifying the K-M model with the Stearns-Noechel(S-N) transfer function. The study used 64 spot colors obtained by mixing a certain series of spot color inks of Siegwerk brand as the target color samples for training, carried out two-color, three-color, and four-color mixed spectrum prediction research respectively, and obtained three quantities of primary color inks when mixed . The optimal paper-based weights are -0.5, -0.1, 0.2, respectively. Bringing the best paper-based weight into the prediction model, and using CIE1976 color difference, CIEDE2000 color difference and root mean square error(RMSE) as evaluation indicators, two, three, and four-color spot colors with known formulas were randomly selected for accuracy testing experiments. The experimental results show that compared with the traditional K-M method, the CIE1976 color difference, CIEDE2000 color difference and RMSE of the method proposed in this study are reduced from 5.748, 3.471 and 0.048 to 2.077, 1.441 and 0.011, respectively. Compared with the traditional K-M method, the prediction accuracy of this method is greatly improved, and it can better realize the spectrum prediction of the mixed color of offset printing ink with higher accuracy.