Retaining needle capable of being closed by virtue of positive pressure and displaying blood back

The invention discloses a retaining needle capable of being closed by virtue of positive pressure and displaying blood back. The retaining needle provided by the invention comprises a vein retaining tube, a seal plug, a flexible connecting tube, an elastic membrane and a positive pressure switch, wherein the downstream end in the liquid flow direction of the flexible connecting tube is connected with the retaining needle; the upstream end in the liquid flow direction of the flexible connecting tube is connected with the positive pressure switch; and the elastic membrane is installed in the positive pressure switch, when the positive pressure switch is switched on, the elastic membrane is in a loose state, and when the positive pressure switch is closed, the elastic membrane is pressed so as to seal a pipeline, simultaneously the liquid in the pipeline of the flexible connecting rod is extruded effectively, a more sensitive blood back indicating function in venipuncture is improved, and a blood release flow-ba