The invention discloses a method of gradually evaporating/condensing to concentrate formaldehyde. The method comprises the following steps: formaldehyde solutions to be concentrated are filled into a formaldehyde concentrating evaporator through a feeding port after heated; in the formaldehyde concentrating evaporator, liquid phase gradual evaporation is carried out on the formaldehyde solutions below the feeding port, and gas phase gradual condensation is carried out on the formaldehyde solutions above the feeding port; and concentrated formaldehyde is obtained on the bottom portion of the formaldehyde concentrating evaporator. The method of gradually evaporating/condensing to concentrate formaldehyde can prepare high concentration formaldehyde water solution with formaldehyde content higher than 60% to 70%, and can prevent paraformaldehyde solid sediment from being generated.