A blood sampling device contains: a body, a fitting sleeve, and an adjusting sleeve. The body includes a lancet, a driving edge for driving the lancet, a control switch for controlling the driving edge, and a connecting section. The fitting sleeve includes a first fitting section, a second fitting section, and a connecting groove, wherein the second fitting section has a coupling portion for connecting with the connecting section. The fitting sleeve further includes a trench, an outlet, and a fixing rib. The adjusting sleeve includes a coupling section, a controlling section opposite to the coupling section, a tilted guiding slot for retaining with the fixing rib, and an exit corresponding to the outlet, The second fitting section of the fitting sleeve has a transparent window for viewing the coupling section of the adjusting sleeve, and the coupling section has a length identification portion injection molded thereon.