A safe simple disposable automatic blood lancet includes a casing, a lancet core, a spring and a protective bar. The ejection cavity is provided inside with a space that allows the push arm to move downwards; the protective bar is provided at the end inside the ejection cavity with a safety action portion; with the protective bar assembled, this safety action portion occupies the space in the X, Y or Z direction, so as to prevent the barb from being self-locked with the self-locking barb with the push arm moving downwards; with the protective bar disassembled, this safety action portion withdraws from the space, so as to allow the barb to be self-locked with the self-locking barb with the push arm moving downwards. The lancet core can be assembled into the locking state of ready for ejection; the blood lancet can be placed into the ejection state just by twisting off the protective bar during application, and into the locking state automatically after ejection so as to be unreusable.