A vacuum drying apparatus of a circulative preheating type comprises a working area, containing a material-filling-preheating sub-area, a heating sub-area, a drying sub-area, and a material-discharging sub-area; a rotatable frame, rotatably installed in the working area and having four accommodating spaces corresponding to the sub-areas of the working area, respectively; four cylinders, installed in the accommodating spaces of the rotatable frame and corresponding to the sub-areas of the working area; and a driving assembly, connected to and thereby driving the rotatable frame to rotate; wherein when the rotatable frame rotates, each of the cylinders is moved from a currently corresponding sub-area to an adjacent said sub-area. Thereby, since the cylinders have been preheated in the material-filling-preheating sub-area before receiving a heating procedure in the heating sub-area, a required duration of the heating procedure can be reduced.