In this paper, Invar alloy composition is described by using binary cluster as basic structural unit. We use “cluster resonance” structure model and establish the connection of a known binary clusters with atoms by an atomic ratio of 1:x to describe Invar alloy component empirical criterion. Invar alloy composition formula is given, that is, Invar alloy component=clusters1 (glue atom)x. Using this criterion, some of Invar alloy compositions are calculated, and it is confirmed that the experimentally determined typical Invar alloy compositions are in good agreement with the compositions calculated from the formula. This shows that the formation of the rule of the Invar alloy composition is well explained by the composition formulae criterion.