The invention forming the concrete slab after installing relates to the underground structure and its construction method, and more particularly, at once sick oyster at least two basements because of the size of earth pressure bearing capacity of the floor beams and beam floor, first to the ground the lowest layer of the building because you can construction is convenient and shorten the construction period, it can even reduce the wall thickness of the underground structures strengthen the soil pressure support structure, and this can follow to reduce the concrete required for the construction of underground structures and spaces of underground structures which can improve the utilization, to a underground structure and the construction method.