Abstract: Detailed investigations on how to choose the optimal double-spike and the optimal mixture ratios of the two single spikes in the double-spike, as well as on how to determinate the optimal sample/spike ratio range during measurement of calcium isotopic compositions by thermal ionization mass spectrometer (TIMS), were carefully carried out in both theory and practices. Compared to the ⁴²Ca-⁴⁸Ca and ⁴³Ca-⁴⁸Ca double-spikes, which are used widely, ⁴²Ca-⁴³Ca double-spike has its distinctive advantages. Firstly, although the error on the fractionation factor α is relatively bigger when using ⁴²Ca-⁴³Ca double-spike, but it only fluctuates a little at a much wide range of sample/spike ratio in spiked samples than using the ⁴²Ca-⁴⁸Ca or ⁴³Ca-⁴⁸Ca. Secondly, the average mass of the two isotopes in ⁴²Ca-⁴³Ca double-spike and target ⁴⁴Ca/⁴⁰Ca ratio, only differs by 0.5 mass unit from each other, thus errors in the corrected ratio resulted from fractionation correction will be minimized, especially when a lower amount of sample was loaded and/or the large fractionation on the instrument occurred during mass determination. Finally, ⁴²Ca and ⁴³Ca can be simultaneously collected by TIMS without significant ion optic effect, which can enhance the data quality collected. In summary, ⁴²Ca-⁴³Ca double-spike should be the optimal one to be used in the double-spike technique for precisely measuring calcium isotopic compositions by TIMS.