The utility model relates to a back-pressure valve for preventing cement mortar from flowing back into a casing from an annulus space after the completion of well cementing construction, in particular to a novel well cementing back-pressure valve. The utility model provides a back-pressure valve, which is compact and simple in structure and is easy to assemble. The back-pressure valve comprises a body (7), wherein the upper end of the body (7) is provided with a casing coupling (1); the body (7) is further provided with a baffle plate (2) and a tray (6); a sealing groove is formed in the baffle plate (2); an O-shaped sealing ring (3) is arranged in the sealing groove; a nylon ball (4) is arranged in a cavity below the baffle plate (2); and an adjusting sleeve (5) is arranged between the baffle plate (2) and the tray (6), and is used for isolating the baffle plate (2) away from the tray (6). After the structure is adopted, the back-pressure valve is simple in structure and is very easy to assemble, great convenience is brought to examination before tools are put into a well, the self-returning capability of the nylon ball in various well deviation states is increased, and high safety is realized.