In recent years, the geochemistry of non-traditional stable isotopes is developing to be a novel studying field. The isotope double spike technique has been becoming one of the main analytical methods in determining isotopic composition. However, the always perplexing problem for researchers is of how to get the best mixing proportion between the two single spikes (A), and the optimal mixing ratio for the double spike and sample (p). These two parameters severely restrict the accuracy, precision and the long reproducibility of the isotopes measurement. In this paper, we systematically elaborated on the mathematical principle of double spike technique as an example of chromium (Cr) and molybdenum (Mo) isotopes, and further discussed the numerical simulation optimization for getting above two parameters combined with the Monte-Carlo error model. The optimal simulated ratios of A and p for Cr-50-Cr-54 and Mo-97-Mo-100 double spike are presented. The case study showed that this numerical simulation method for double spike can be applied to all elements with 4 or more isotopes. On the basis of the above, we presented the detailed description of how to use the numerical optimization results of double spike to select and calibrate double spike. Meanwhile, some trouble issues and the basic routine during using double spike are also presented for researchers who will engage in double spike. This paper will be helpful to promote the development of double spike and its extensively application in Chinese geosciences community.