The invention provides an internal pressurization type power mandrel and a downhole power device used for oil test completion. The power mandrel comprises a mounting hole, a pressure transmitting hole, a fixing groove and a sealing piece mounting groove, wherein a rupture disc can be mounted in the mounting hole; and the pressure transmitting hole and the mounting hole can transmit the pressure inside the power mandrel. The device includes an outer cylinder, the power mandrel, the rupture disc, a fixing piece, a first sealing piece, a second sealing piece and a third sealing piece, wherein thepower mandrel is arranged in an inner cavity of the integrated outer cylinder, the outer wall of the power mandrel is provided with a first sealing groove, the pressure transmitting hole, the fixinggroove, a second sealing groove and a third sealing groove which are sequentially distributed in the axial direction, and a closed first annular cavity space and a closed second annular cavity space can be formed among the power mandrel, the rupture disc and the outer cylinder correspondingly. The downhole power device has the beneficial effects that the external integration of a downhole tool canbe realized, and the change of a tool state can be met.