A retained needle structure of a comprises a syringe needle with one or more fastening portion, a catheter with a cannula having a flexible tube, as well as a syringe cover with an aperture and gliding portion. The syringe needle has an inserting portion which is axially operated by users to enhance safety practice in clinical use. First design is the sliding slot has a deflected slant portion at the distal end. When the fastening portion is at the rear end of sliding slot, the injection needle is already inside of syringe cover. At the same point, body of the syringe needle is deflected laterally from the center of the syringe cover.The second design is used the levering effect from gravity of syringe cover. When the fastening portion is at the rear end of sliding slot, the syringe cover is vertically rotated from the original center of the syringe cover.