The invention relates to a double-drum isothermal conversion hydrogen production method and an isothermal conversion furnace. The crude gas from a pulverized coal gasification apparatus is subjected to condensate separation, then is subjected to heat exchange, detoxification, and water-gas ratio adjustment, then enters a first reaction cavity of the isothermal conversion furnace for carrying out amedium-temperature conversion reaction and by-producing a medium-pressure saturated steam, then enters a second reaction cavity for carrying out a low-temperature conversion reaction to produce crudehydrogen with a hydrogen dry base content of 50-65 v% and by-producing a low-pressure saturated steam. The medium-pressure saturated steam returns a first drum and then is divided into two strands after liquid separation, the first strand of the medium-pressure saturated steam as a process steam is supplemented into the crude gas discharged from a crude gas preheater to adjust the water-gas ratio, and the second strand of the medium-pressure saturated steam is sent out of the boundary area. The low-pressure saturated steam is subjected to heat recovery, and then is sent to downstream users. The secondary conversion gas continues to recover heat, and is sent to a downstream purification apparatus as crude hydrogen after the temperature drops to 180 DEG C-200 DEG C.