The invention relates to a method for improving the oil deposit recovery ratio of deep massive extremely-viscous oil in an extraction mode of gravity drainage assisted steam flooding. In the method, superposition horizontal wells are arranged among wells by combining results of researches on the fine contrast of small geological strata of massive oil deposit, the distribution rule of interlayer, the distribution rule of remaining oil and the like and a fine research on numerical simulation; an upper superposition horizontal well is used for injecting steam, a peripheral vertical shaft is used for production, and a lower superposition horizontal well is used for discharging liquid (crude oil between condensate water and the horizontal wells); and thus, a production-injection ratio of high-speed steam injection of deep oil deposit is improved, the dryness of steam on the well bottom is improved, the pressure of an oil layer is reduced, a steam cavity is easy to expand, the swept volume is improved, a development effect is improved, and the aim of improving the recovery ratio finally is fulfilled.