A slide rail bracket assembly includes a slide rail bracket including a bracket body and a positioning member affixed to one side of the bracket body, and a fastening device including a hook member that includes two hook bases affixed to an opposite side of the bracket body, two spring arms respectively extended from the hook bases, and a hooking portion connected between the spring arm, and a release member that includes two actuation portions respectively pivotally connected to the spring arms of the hook member, two bearing portions respectively extended from respective one ends of the actuation portions and two pull-off portions respectively extended from respective opposite ends of the actuation portions. Operating the release member to bias the actuation portions relative to the hook member can force the bearing portions into abutment against the bracket body and move the spring arms in direction away from the bracket body.