The invention relates to a method for melting and polycondensation reaction of falling film between rows of tubes. The melted prepolymer flows downward from the pores between the film cloth plate and the tubes, and forms a waterfall falling film flow between the rows of U-shaped tubes for melting. In the polycondensation reaction, the melt slides from the tubes, gathers at the bottom of the reactor, is stirred and homogenized by the agitator, and then discharged. The reactor for implementing the method is an integral vertical structure, including a shell, a heat medium box at the upper end and a material box connected thereto, a U-shaped pipe, a bottom shell at the lower end, a stirrer, and the like. The U-shaped tubes are hung vertically, arranged in rows, and evenly pass through the pores of the membrane cloth, and the ports of the U-shaped tubes are connected with the heat medium box for the flow of the heat medium. This reactor has the advantages of small flow resistance, no dead zone, large film forming area, fast surface renewal, uniform and controllable residence time, and meets flexible production. It is suitable for the production of polymers such as polyethylene terephthalate .