A mathematic formula for relationship between trace element abundances in multistage magma fractional crystallization is proposed based on the study of geochemistry of the Emeishan Basalt and relevant basic-ultrabasic layered intrusions. This paper discusses the mechanism of normal and inverse evolution of trace elements and the abundance relation constant (R) in the relationship formula. For some elements the constant (R)varies considerably at different stages of magma fractional crystallization, which gives eyidences for the subdivision of these stages. The abundance relation formula could also be applied to igneous rocks formed by fractional crystallization ofmagmas with different compositions. There are sufficient data of basalts,basaltic-andesites, andesites, dacites and rhyolites in the Andes, Chile cited as instance.