In this paper, the crossover statistical associating fluid theory and Boublik-Alder-Chen-Kreglewski (SAFT-BACK) equation of state (EOS) is established. The system-dependent parameters for pure carbon dioxide and water are regressed and their thermodynamic properties are calculated. Then the classical and crossover SAFT-BACK EOSs are compared. For pure carbon dioxide, their calculated deviations are both small. For pure water, the calculated accuracy with the crossover EOS is better than that with the classical EOS. Then some rules about the crossover method are revealed.