Abstract: Pseudocritical region is generally considered as a region where fluid property has a sharp variation near the pseudocritical point under supercritical conditions.The flow and heat transfer mechanism behaves uniquely and traditional subcritical formulas may not be applicable in this area.Many researchers have devoted to investigate its peculiarities.The pseudocritical region is similar to the two-phase subcritical saturation region.However,the pseudocritical region is a vague concept without specific physical definition,which can not borrow and learn the subcritical concepts.In this study,the pseudocritical region is considered as the secondary phase transition process and the point where the surface tension disappears represents the start of the phase change.Moreover,as the phase transition process satisfies the Ehrenfest equation,the end point of phase transition is determined in combination with the start point.The pseudocritical region based on this method could help and make contribution to the investigation of flow and heat transfer mechanism in this area.