The present invention provides a diaphragm pump of constant pressure type, mainly wherein a passage tube links between said pressure discharge port and said inlet port of said pump cover body; Thereby, the extra water pressure coming from said compressed chamber can be orderly discharged through said pressure discharge port, next guided through said passage tube to reach said inlet port, then directly flowed into said low pressure chamber; Therefore, a close water circulation loop of discharge pressure is built by means of connecting said compressed chamber, said pressure discharge port, said passage tube, said inlet port and said low pressure chamber; Thus, the present invention not only provides the function of keeping constant pressure, but also completely recycles the drained water by extra water pressure without any waste in water resource; Moreover, the present invention not only has the energy-saving effect in power consumption, but also has the environment-protecting effect in reducing operation noise; Thus, it is really an innovative invention with multiple practical effect.