A liquid infusion device comprises a bladder assembly and a flow-regulating portion. The bladder assembly comprises a tubular outer shaft, an inner shaft slidably received within the outer shaft and a bladder covering the outer and inner shafts. The bladder can inflate in both its radial and axial directions whereby reducing the residual amounts of a liquid drug in the bladder on dispensation of the liquid drug. The flow-regulating portion comprises a pipe having at least one small hole or a pipe having a very small diameter, so that the accurate regulation of the liquid drug can be performed.