The experimental data of CFP B40 of rare earth ions in RX compounds is analyzed. On this basis, we propose a new relation, through which the CPP B40 values in RX compounds can be calculated by the simple point charge model, namely, B40=k3(B40)p + c3 The CFP B40 values in a lot of RX compouuds were determinded by this relation. Results are in good agreement with experimental values. Finally, the meaning of the relation are discussed.