The utility model belongs to the technical field of water injection well machinery in oil exploitation, and discloses a constant pressure and quantification water discharging device of a water injection well. A bend and a hoop short section are in threaded connection. An adjustable water hole is installed into an inner hole in the other end of the bend and is in threaded connection with the inner hole. The adjustable water hole is provided with an adjustable nut, a movable baffle and a fixed baffle. The adjustable nut is connected with the movable baffle and the fixed baffle. The constant pressure and quantification water discharging device of the water injection well has the advantages of being directly installed on a valve station of a wellhead casing pipe of the water injection well, and being capable of discharging liquid in the water injection well in a constant pressure and quantification mode according to the design requirements. The constant pressure and quantification water discharging device of the water injection well is convenient to install and easy to manufacture.