To develop a new technology for the purification of formaldehyde from formalin by a distillation method, the vapor-liquid equilibria of formaldehyde water binary systems and the vapor-liquid equilibria of formaldehyde water systems in the presence of diluent were investigated. It was shown by calculation that in some cases, when the concentration of diluent is high, the volatility of formaldehyde is always higher than that of water and that these calculations are in good agreement with experimental data. Based on this principle, the purification of formaldehyde by an extractive distillation method was established.