The work aims to propose a spectral density distribution curve to predict the random proportion of spot color ink. According to the principle of density additivity, the spectral density distribution curves of the primary color of spot color and the diluted primary color would not be intertwined, but move parallel to each other with the increase of spot color proportion (the measured spectral density was solid density). According to Lambert theory, the spectral densities of different concentration were analyzed. After the spectral density coefficient was calculated, the spot color samples with the proportions of 0.1, 0.2, 0.3, 0.4 and 0.5 were made with spot color NZSR and transparent diluents, and the final results were measured. The measured results showed that, the spectral density curve of the same spot color proportion had little difference between the predicted curve and the measured curve. Theoretical and experimental results show that, the proposed method has high prediction accuracy. At the same time, conversion methods between spectral density and spectral reflectance, relative measurement and absolute measurement of spectral data are proposed. To obtain the optimal prediction effect, the concept of maximum proportion and the technical requirements for measuring spectral density distribution are also proposed.