The invention provides a balance piston type constant pressure valve. The balance piston type constant pressure valve comprises a valve body; the valve body is provided with a fluid inlet and a fluidoutlet; a first piston with a piston rod is slidingly assembled in the valve body in a guide mode; the first piston is provided with a piston inlet and a piston outlet communicating with the fluid inlet and the fluid outlet; the valve body is provided with a communicating pipe for communicating a rod cavity with a rodless cavity on two sides of the first piston; the piston rod of the first pistonextends out of the valve body, and is connected with a constant force mechanism; the constant force mechanism comprises a shell fixedly connected to one side of the rod cavity of the valve body; the shell is provided with an atmospheric communicating port and a shell bottom connected with the valve body; a second piston is slidingly assembled in the shell in a guide mode; the second piston and thepiston rod are in jacking fit; and the second piston and the shell bottom form a vacuum state. The balance piston type constant pressure valve has the following beneficial effects: an adjusting spring is not depended; the constant force mechanism is adopted to improve the use stability of the constant pressure valve; the fluid output pressure is precise; and along with automatic adjustment of thefirst piston in use, the fluid output pressure is always maintained stable.