A safety lancet for taking blood has a hollow barrel with two first slots and two second slots and a detachable needle hub. The needle hub is mounted in the barrel and has a moving base with a needle near a proximal end and a positioning base near a distal end. The positioning base detachably connects to the moving base and has a push button and two stopping resilient elements protruding symmetrically from an outer surface of the positioning base and protruding in the first slots before the push button being pushed and protruding in the second slots after the push button being pushed, so the position base can not move backward. Therefore, medical personnel are able to distinguish easily if the lancet has been used or not.