Based on the equation of element abundance relationship between cumulate and residual magma during fractional crystallization of magma, this paper prososes the principle and method for the mutual prediction of compositional trend lines of consanguineous cumulate rocks and magmatic rocks formed at the same crystallization stage. If the abundance constant R (the slope of the compositional trend line of magmatic rock) and the bulk partition coefficient D are known, the compositional trend line of the relevant cumulates could be predicted and vice versa.1. When R> 1, the compositional trend line of cumulates lies to the lower right side of the trend line of relevant magmatic rocks.2. When R<0, the compositional trend line of cumulates lies to the lower left side of the trend line of relevant magmatic rocks.3. When 0