Milk is considered to be one of the most perfect natural foods for human beings. The analysis of effective components and determination of important components in milk is an important practical work. Based on the previous measurement data of fat volume concentration and refractive index of milk in the market, this paper forms a fitting formula through mathematical fitting, which can solve the corresponding relationship between nonmeasurement points of milk concentration and refractive index. Because the actual measurement points are limited, its practical application is not convenient. However, fitting formulas can calculate the corresponding values of infinitely many points, which can be easily applied. However, judging from the goodness of fit of the fitting formula, a theoretically good fitting formula may produce underfitting and overfitting in practical application. Through the analysis of the first-order to sixth-order fitting formulas of milk concentration and refractive index, this paper tells us that the fitting formula with high goodness of fit is not necessarily a good formula. The specific problems should be analyzed in the application process to make the theory more consistent with the actual situation. From the calculation and analysis in this paper, it can be seen that the fitted sixth-order formula has the best effect when the fat volume concentration of milk is 0–10%. When considering a fat volume concentration of 0–34% in milk, the fitted second-order formula performs better.