The utility model relates to a diameter changeable valve. The diameter changeable valve is characterized in that a diameter changeable pipe which is fixed on a valve body by using screws is installed at one end of the valve body on which a valve plate is installed through a valve seat, and a flange with a back tapered sealing ring is arranged on the small diameter pipe of the diameter changeable pipe and is capable of moving on the small diameter pipe. The utility model has the advantages that the installed diameter changeable pipe can make the diameter changeable valve have the characteristic of changing the diameter, the back tapered sealing ring on the small diameter pipe of the diameter changeable pipe can make the flange capable of making back and forth movement on the small diameter pipe to achieve the telescopic purpose, and the diameter changeable valve has the function of self-regulation of the flow rate. In addition, the utility model also has the advantages of convenient replacement and maintenance, no leakage, and easy operation.