A drug delivery device mainly has a housing, which can be easily held and operated. The housing has one end formed with an adjustment seat capable of adjusting a range distance, and the other end connected to a gas pressure control source for providing mainstream and substream gas pressures. Disposed in the housing are a drug delivery pressuring tube, an embedded drug-can connecting seat or an external drug-can connecting seat, a replaceable drug-can container and a gas communication tube. According to this design, the minor liquid drug can be controlled, the drug-can can be replaced and the continuous quantitative injection of drug can be made. Also, all damaged or dirty members of the invention may be disassembled, cleaned or replaced.
