A blow molding device for a rotary bottle blowing machine includes: a support base including a power-input shaft and a mold-insert control shaft which are pivotally disposed on the support base and extend along an axial direction; a drive source for driving the power-input shaft and the mold-insert control shaft to rotate; and a mold assembly with a first and second movable mold inserts pivotally disposed on the support base. The mold assembly makes the first and second movable mold inserts pivot toward or away from each other, rotation of the power-input shaft makes a bottom mold unit move in the axial direction, and can lock the first and second movable mold inserts. A method for using the blow molding device includes steps of mold opening or closing, mold lifting, and mold locking, and these steps are performed by using a single drive source.