A safe venous blood sampling system with a self-locking device consists of a safe blood sampling needle, a needle holder and a vacuum blood sampling tube jointly. An inner hole of a conical head at the front end of an outer barrel is communicated with an inner cavity of the outer barrel. Three outwards unfolding ribs are arranged on the lower section of the safe venous blood sampling system, the largest diameters of the three outwards unfolding ribs are equal to or smaller than the inner diameter of the outer barrel, pitches among the three outwards unfolding ribs are larger than the width of two inserts, one of theribs is shortened, the bottom of the rib can be supported and locked by the bottom edge of a window on the front side wall of the outer barrel, a spring is compressed in the inner hole of the conical head, and the outer side of the window of the outer barrel is covered by a key contact plate. After blood is sampled, a triggering key is pressed down, the rib is not locked by the bottom edge of the window again, a needle seat is released, the spring resets, a needle tube is driven to retract into the inner cavity of the outer barrel, and a needle tip automatically deviates and is prevented from being exposed again. The inserts which are inclined inwards and forwards are arranged on the side wall of the front section of the outer barrel, an annular bump is disposed on the front section of an end cap of the blood sampling tube, a plurality of arc-shaped bumps are arranged at the lower end of the end cap of the blood sampling tube, the annular bump can be in embedded locking by the insert, the arc-shaped bumps rotatably support to release embedded locking of the insert, the blood sampling tube can be conveniently locked and released, so that blood sampling is safer and reliable, after being used once, the safe venous blood sampling system is destructed, and the needle tip retracts automatically, so that medical staff is protected against needling injury.