A safety injection device, comprising at least: a needle , with one end connecting to an injection as a proximal end and the other end as a distal end ; a needle hub , an elastic member , a slider , a needle shield , and a protective sleeve being sleeved along the direction from the proximal end to the distal end of the needle in sequence; the needle hub is sleeved on the needle with the needle as a shaft, the needle hub includes a base and a rod , and the rod includes a rod core and an extension portion ; a proximal end of the elastic member is sleeved on the rod , a distal end of the elastic member being in contact with a proximal end of the slider ; the slider has a guide portion , the distal end of the guide portion is fitted into the inside of the needle shield , and the proximal end of the guide portion is fitted into the distal end of the elastic member ; one end of the first inclined surface has a blocking portion ; the protective sleeve is provided outside the needle shield , and an inner wall of the protective sleeve being in contact with an outer wall of the needle shield and an outer wall of the slider ; wherein the slider is configured to that, when the distal end surface of the needle shield is pressed, the slider rotates under the limitation of the guide portion with the needle as a shaft, and the slider is locked at the blocking portion at one end of the first inclined surface , and to rotate irreversibly after a single use, the limiting protrusion of the slider falls into a restricting area and cannot return to a state of being in contact with the first inclined surface , characterized in that the proximal end of the slider comprises a through hole and the shapes of the extension portion and the through hole do not match and interfere after irreversibly rotating the slider , thereby restricting the movement of the needle shield towards the slider 2 which no longer causes a deformation of the elastic member . a needle , with one end connecting to an injection as a proximal end and the other end as a distal end ; a needle hub , an elastic member , a slider , a needle shield , and a protective sleeve being sleeved along the direction from the proximal end to the distal end of the needle in sequence; the needle hub is sleeved on the needle with the needle as a shaft, the needle hub includes a base and a rod , and the rod includes a rod core and an extension portion ; a proximal end of the elastic member is sleeved on the rod , a distal end of the elastic member being in contact with a proximal end of the slider ; the slider has a guide portion , the distal end of the guide portion is fitted into the inside of the needle shield , and the proximal end of the guide portion is fitted into the distal end of the elastic member ; one end of the first inclined surface has a blocking portion ; the protective sleeve is provided outside the needle shield , and an inner wall of the protective sleeve being in contact with an outer wall of the needle shield and an outer wall of the slider ; wherein the slider is configured to that, when the distal end surface of the needle shield is pressed, the slider rotates under the limitation of the guide portion with the needle as a shaft, and the slider is locked at the blocking portion at one end of the first inclined surface , and to rotate irreversibly after a single use, the limiting protrusion of the slider falls into a restricting area and cannot return to a state of being in contact with the first inclined surface , characterized in that the proximal end of the slider comprises a through hole and the shapes of the extension portion and the through hole do not match and interfere after irreversibly rotating the slider , thereby restricting the movement of the needle shield towards the slider 2 which no longer causes a deformation of the elastic member . <EOS>