An auto-injector for delivering a liquid medicament comprising: a housing arranged to contain a syringe with a piston for sealing the syringe and displacing the medicament, the housing having a distal end and a proximal end intended to be applied against an injection site； and a telescopic plunger unit arranged between the piston and the distal end of the housing, the telescopic plunger unit comprising a telescopic frame and a spring element surrounding a plunger element, wherein the plunger element is fixed to a base of the telescopic plunger unit and the spring element is held in an initial compressed state by a compressive force exerted by the base of the telescopic plunger unit； wherein when the spring element is released, a tapering profile of the telescopic frame is configured to provide varying friction with extension such that resistance to telescopic extension decreases as spring force provided by the spring element decreases with increasing extension of the telescopic plunger unit.一種用於輸送液體藥劑的自動注射器，包括：殼體，該殼體佈置成包含注射筒，該注射筒具有用於密封該注射筒並且排出藥劑的活塞，該殼體具有遠端和近端，該近端旨在施用在注射部位上；以及伸縮式柱塞單元，該柱塞單元佈置在該活塞和該殼體的遠端之間，該伸縮式柱塞單元包括伸縮式框架和圍繞柱塞元件的彈簧元件，其中該柱塞元件固定到該伸縮式柱塞單元的底部，該彈簧元件通過該伸縮式柱塞單元的底部施加的壓縮力保持在初始壓縮狀態；其中當該彈簧元件被釋放時，該伸縮式框架構造為漸縮輪廓，用以提供隨著伸長變化的摩擦，使得當該彈簧元件提供的彈簧力隨著該伸縮式柱塞單元不斷伸長而減小時，針對可伸縮伸長的阻力減小。
