An auto-retractable safety blood collection needle, comprising: a needle holder comprising a first tube, a second tube and a holder tail connected in sequence; a needle; a built-in sliding sleeve which is elongated and through and has a C-shaped cross section; a needle shield which is hollow and elongated; and a spring; characterized in that a diameter of the second tube is larger than a diameter of the first tube; a rear of the needle is mounted in the first tube; the first tube and the second tube both are installed in the built-in sliding sleeve, and the second tube is fitted with a built-in slider to allow the second tube to slide back and forth; a holder tail of the needle holder is located outside a rear of the built-in sliding sleeve which is mounted in the needle shield and is capable of sliding back and forth; the holder tail is allowed to be locked at or unlocked from a rear of the needle shield; the spring is sheathed on the first tube; a front end of the spring abuts against an inner wall of the needle shield, and a rear of the spring abuts against a front end of the second tube; a first locking mechanism is provided between the needle holder and the built-in sliding sleeve and is configured for locking the second tube with the built-in sliding sleeve; a second locking mechanism is provided between the built-in sliding sleeve and the needle shield and is configured for locking the built-in sliding sleeve with the needle shield; a third locking mechanism is provided between the holder tail and the needle shield and is configured for locking the holder tail with the needle shield; when the holder tail is locked at a rear of the needle shield, the spring is in a compressed state, and a front end and a rear of the built-in sliding sleeve are respectively limited by the inner wall of the needle shield and the holder tail; when the holder tail is unlocked from the rear of the needle shield, the spring forces the second tube to slide rearward to a locking position of the built-in sliding sleeve; the needle holder keeps sliding rearward, and the built-in sliding sleeve is driven to slide rearward to a locking position of the needle shield; at this time, the needle is fully covered by the needle shield, and a tip of the needle is not exposed. a needle holder comprising a first tube, a second tube and a holder tail connected in sequence; a needle; a built-in sliding sleeve which is elongated and through and has a C-shaped cross section; a needle shield which is hollow and elongated; and a spring; characterized in that a diameter of the second tube is larger than a diameter of the first tube; a rear of the needle is mounted in the first tube; the first tube and the second tube both are installed in the built-in sliding sleeve, and the second tube is fitted with a built-in slider to allow the second tube to slide back and forth; a holder tail of the needle holder is located outside a rear of the built-in sliding sleeve which is mounted in the needle shield and is capable of sliding back and forth; the holder tail is allowed to be locked at or unlocked from a rear of the needle shield; the spring is sheathed on the first tube; a front end of the spring abuts against an inner wall of the needle shield, and a rear of the spring abuts against a front end of the second tube; a first locking mechanism is provided between the needle holder and the built-in sliding sleeve and is configured for locking the second tube with the built-in sliding sleeve; a second locking mechanism is provided between the built-in sliding sleeve and the needle shield and is configured for locking the built-in sliding sleeve with the needle shield; a third locking mechanism is provided between the holder tail and the needle shield and is configured for locking the holder tail with the needle shield; when the holder tail is locked at a rear of the needle shield, the spring is in a compressed state, and a front end and a rear of the built-in sliding sleeve are respectively limited by the inner wall of the needle shield and the holder tail; when the holder tail is unlocked from the rear of the needle shield, the spring forces the second tube to slide rearward to a locking position of the built-in sliding sleeve; the needle holder keeps sliding rearward, and the built-in sliding sleeve is driven to slide rearward to a locking position of the needle shield; at this time, the needle is fully covered by the needle shield, and a tip of the needle is not exposed.