A catheter introducer assembly comprising: a needle comprising a longitudinal axis, a proximal end, a sharp distal end and an outer surface; a catheter assembly comprising a catheter adapter and a catheter tube coaxial with the needle; a needle shield assembly comprising: a first housing for shielding a sharp distal end of the needle when deployed in a shielding position, the first housing comprising an outer wall and an internal lumen and slidable along the outer surface of the needle;a spring a second housing covering the spring and at least part of the first housing; and a blocking object, characterized in that the spring is disposed on the first housing and exerts a biasing force on the blocking object; the first housing is provided with an opening therein for carrying the needle blocking object, wherein the opening extends from the outer wall to the internal lumen; the second housing is provided with aside hole therein; the catheter adapter is provided with a detent on an inner surface thereof; in a non-shielding position the opening maintains the blocking object in contact with the outer surface of the needle and a portion of the blocking object protrudes through the side hole and the opening and engages with the detent; in the shielding position, the opening allows the blocking object to enter the internal lumen at least partially, and disengage from the detent, thereby blocking distal movement of the needle and releasing the needle shield assembly from the catheter assembly. a needle comprising a longitudinal axis, a proximal end, a sharp distal end and an outer surface; a catheter assembly comprising a catheter adapter and a catheter tube coaxial with the needle; a needle shield assembly comprising: a first housing for shielding a sharp distal end of the needle when deployed in a shielding position, the first housing comprising an outer wall and an internal lumen and slidable along the outer surface of the needle;a spring a second housing covering the spring and at least part of the first housing; and a blocking object, a first housing for shielding a sharp distal end of the needle when deployed in a shielding position, the first housing comprising an outer wall and an internal lumen and slidable along the outer surface of the needle;a spring a second housing covering the spring and at least part of the first housing; and a blocking object, characterized in that the spring is disposed on the first housing and exerts a biasing force on the blocking object; the first housing is provided with an opening therein for carrying the needle blocking object, wherein the opening extends from the outer wall to the internal lumen; the second housing is provided with aside hole therein; the catheter adapter is provided with a detent on an inner surface thereof; in a non-shielding position the opening maintains the blocking object in contact with the outer surface of the needle and a portion of the blocking object protrudes through the side hole and the opening and engages with the detent; in the shielding position, the opening allows the blocking object to enter the internal lumen at least partially, and disengage from the detent, thereby blocking distal movement of the needle and releasing the needle shield assembly from the catheter assembly.