Abstract: A cooler-dehumidifier-filter 3-in-one apparatus for treating compressed air, comprising an air inlet-outlet mechanism with a built-up radiator cisterns connected therebelow, having an air diffusion mechanism and an air filtering mechanism set therein to communicate with the air intake conduit so as to provide initial cooling and filtering effect; an exhausting device being made on the bottom of said air filtering mechanism to produce spiral air flow so as to dehumidfy the compressed air; the dehumidified air being to soar upward to pass through said filtering mechanism and the interlocking holes of the flange of said air diffusion mechanism to further be squeezed and diffused to cool down again; said air current being further to pass through said upwardly disposed filtering cylinder for secondary filtration; a water cooling effect being achieved by means of the arrangement of water inlet and outlet on said radiator cisterns to match the communicating pipe formed by the vertical conduits of said radiator ciste