A tension unit for a drum set pedal has a mounting shell, a lower locking nut, an upper locking nut, a threaded shaft and a spring. The mounting shell has an inner chamber, a mounting hole, at least one positioning protrusion and a locking ring. The positioning protrusion and locking ring are respectively formed on a top and bottom face of the mounting shell around the mounting hole. The lower locking nut has a threaded hole and an engaging ring engaging the locking ring. The upper locking nut abuts the top face of the mounting shell and has a threaded hole, a position ring engaging the positioning protrusion and an annular groove allowing the upper locking nut to flex. The threaded shaft is mounted through the mounting hole and the locking nuts and engages the locking nuts. The spring is connected to the threaded shaft.