An adjustable forearm crutch, wherein the fork support is formed with a central reinforcing rib for increasing the bending-resistant strength of the fork support, the reinforcing rib and the adjacent wall of the fork support defining a recess. After the fork support is molded, the recess can serve as a standard face for the processing procedure so that when punching holes on the fork support, no deflection will take place. The fork support and fixing tube being inserted in through holes of a sleeve member so that the waste chips and water produced when processing the fork support can be easily removed. The through hole of the sleeve member for receiving the fixing tube is formed with an annular groove which provides a resilient clamping effect, the end of which is cut into several equal sections for providing resilient clamping effect, so that the adjusting tube is firmly supported at two points without loosening or swinging. When the adjusting tube is installed into the fixing tube, the fastening member of the adjusting tube is guided and compressed by an arched face of the sleeve member to facilitate the installation of the adjusting tube into the fixing tube.