The invention discloses a safety pin which comprises a pin body, a pin sleeve and a syringe, wherein the pin sleeve is matched with the pin body; the syringe is connected with the pin body; the pin sleeve is axially provided with a chute; the chute comprises a longitudinal slot and a lateral slot which are mutually communicated; the lateral slot comprises a positioning slot and a clamping slot, wherein the positioning slot and the clamping slot are positioned on two sides of the bottom end of the longitudinal slot; one end of the pin body, which approaches to the syringe, is provided with a sliding block; the sliding block is suitable for sliding along the chute; one end of the sliding block, which faces to the clamping slot, is less than one end of the sliding block, which faces to the positioning slot; at least one of the upper side and the lower side of the opening end of the positioning slot, which faces to the longitudinal slot, is provided with a bulge; therefore, the sliding block can conveniently pass through the bulge to enter the longitudinal slot; once the sliding block slides out of the positioning slot, the sliding block can not pass through the bulge to go back to the positioning slot because one end of the sliding block, which faces to the positioning slot, is bigger; and therefore, the safety pin is not suitable to use again.