It is a kind of for conveying the automatic injector of liquid medicine, comprising: shell is arranged to accommodate the injection tube with piston, and for sealing the injection tube and shifting the drug, the shell has proximal end and is intended to be applied against the distal end of injection site And scalable plunger unit, it is arranged between the piston and the proximal end of the shell, the scalable plunger unit includes Extensible framework and the spring element around piston element, wherein the piston element is fixed to the base portion of the scalable plunger unit, and the spring element is maintained at initial compressed state by the compressing force of the base portion application by the scalable plunger unit Wherein when the spring element is released, the tapered profiles of the Extensible framework are configured to provide the frictional force changed with extension, so that the flexible resistance that extends reduces with the spring force reduction provided by the spring element with the increased extension of the scalable plunger unit.
