The invention provides a balance piston type constant pressure valve which comprises a valve body, wherein a fluid inlet and a fluid outlet are formed in the valve body; a first piston with a piston rod is in guiding and sliding fit with the interior of the valve body; a piston inlet and a piston outlet which are used for communicating the fluid inlet and the fluid outlet are formed in the first piston; a communicating pipe used for communicating a rod cavity and a rodless cavity on the two sides of the first piston is arranged on the valve body; the piston rod of the first piston extends out of the valve body and is connected with a constant force mechanism; the constant force mechanism comprises a shell which is fixedly connected to one side of the rod cavity of the valve body; the shell is provided with a atmosphere communicating opening and a shell bottom connected with the valve body; a second piston is in guiding and sliding fit with the interior of the shell; the second piston is in pushing fit with the piston rod, and vacuum is formed between the second piston and the shell bottom. The balance piston type constant pressure valve has the benefits as follows: the balance piston type constant pressure valve is independent of a regulating spring, the using stability of the constant pressure valve is improved by adopting the constant force mechanism, the fluid delivery pressure intensity is accurate, and the fluid delivery pressure intensity keeps stable throughout along with the automatic regulation of the first piston.