The invention relates to a disposable a safety syringe, which mainly includes a needle, a protective sleeve, and a barrel, the needle is attached to the barrel by a plug-connection portion, and on theouter edge of the plug-connection portion is disposed a slide block which consists of a basel, a guide-moved rod, and an upper stop portion, and at the outer edge of the protective sleeve is disposedasliding rail in which is disposed a protruding point near the front end and back end on one side or both side, and one or more resist-back block is disposed behind the protruding point of the sliding rail near its back end, the protective sleeve is covered on the outer edge of the needle, the needle, the needle pass through the sliding rail of the protective sleeve by the guide-moved rod, and ispressed against the interior side wall face of the protective sleeve by the base, and is pressed against the outer side wall face by the upper stop portion.