An injection pen includes an activation mechanism and a driving mechanism. The needle-sheath coupling link and the front spring of the starting mechanism are installed in the front casing and the inner casing of the inner casing, and the front cover is disposed at the front end of the front casing and the cover needle sheath After the driving mechanism, the housing and the inner driving sleeve are connected to the front housing, the trigger tube and the key board are installed in the driving sleeve, and the spring is installed in the trigger tube after the push rod is combined, and the elastic baffle card of the driving shell is driven. The buckle drive sleeve and the push rod lock the rear spring, and the injection needle set can be installed between the activation mechanism and the drive mechanism, and the front cover is removed, and the needle sheath can press and contract into the front housing to expose the needle, and the retracted needle The sheath drives the connecting rod to push open the elastic baffle of the driving sleeve to unlock, and the released rear spring pushes the push rod to transmit the medicament to the injection needle set. After the injection, the needle sheath is pushed forward by the front spring, and the button is pulled through the connecting rod. The card is fastened in the inner casing, and the needle sheath cannot be pressed and retracted, so that the needle after use is protected from accidents caused by needle sticking.一種注射筆，包含啟動機構與驅動機構，啟動機構之針鞘結合連桿與前彈簧裝設於前殼體及其內部的內套殼中，前蓋設於前殼體前端而罩蓋針鞘，驅動機構之後殼體與其內部的驅動套殼連接前殼體，扳機管與鍵板裝設於驅動套殼中，推桿結合後彈簧裝設於扳機管內，驅動套殼的彈性擋板卡扣驅動套殼與推桿而鎖定後彈簧，啟動機構與驅動機構之間能裝設注射針組，取下前蓋，針鞘能按壓內縮至前殼體中而露出針頭，內縮的針鞘帶動連桿推開驅動套殼的彈性擋板解鎖，被釋放的後彈簧推動推桿，使注射針組傳輸藥劑，注射後，針鞘為前彈簧推抵復位，並通過連桿抽拔鍵板卡扣於內套殼中，針鞘即無法被按壓內縮，讓使用後的針頭被保護，避免人們被針扎之意外。
