A cup holder fastening structure includes a cup body, a fastening means hinged on the cup body and a pivotal unit. The cup body has a first coupling portion extended therefrom. The fastening means includes a strap containing a latch teeth section and a holder. The strap is fastened to an upper end of the holder. The holder has a slot to be threaded through by the strap and a second coupling portion extended from one side to mate the first coupling portion. The pivotal unit is hinged between the first and second coupling portions to adjust the angle of the cup body. Through the strap the invention can be strapped on any type of vehicle bars and installed and removed easily. Through the pivotal unit the cup body can be adjusted to a desired use angle to provide greater usability and practicality.