The present invention relates to a method for mass preparation of granular polycrystalline silicon in a fluidized bed reactor, comprising (a) a reactor tube, (b) a reactor shell encompassing the reactor tube, (c) an inner zone formed within the reactor tube, where a silicon particle bed is formed and silicon deposition occurs, and an outer zone formed in between the reactor shell and the reactor tube, which is maintained under an inert gas atmosphere, and (d) a controlling means to keep the pressure difference between the inner zone and the outer zone being maintained within the range of 0 to 1 bar, thereby capable of maintaining physical stability of the reactor tube and efficiently preparing granular polycrystalline silicon even at a relatively high reaction pressure.