The invention belongs to the technical field of water electrolysis, and relates to a device and method for producing hydrogen through step-by-step water electrolysis based on an all-vanadium liquid flow redox medium The device comprises two diaphragm electrolytic cells (a cell 1 and a cell 2), a hydrogen evolution unit and an acidic all-vanadium electrolyte. According to the method, the water electrolysis is divided into two steps, charging an all-vanadium redox flow battery and producing hydrogen and oxygen. According to the first step, VO < 2 + > in an anode chamber in a tank 1 is oxidized into VO < 2 + >, and the VO 2 <+> is introduced into a tank 2; and V < 3 + > in a cathode chamber in the tank 1 is reduced into V < 2 + >, and meanwhile, the V < 2 + > is introduced into the hydrogen evolution unit. According to the second 2, in a tank 2, VO 2 <+> in the cathode chamber is reduced into VO < 2 + > and the VO < 2 + > is circulated back to the tank 1, and meanwhile, the anode generates oxygen; and V < 2 + > is oxidized into V < 3 + > in the hydrogen evolution unit and the V < 3 + > is circulated back to the tank 1, and hydrogen is generated at the same time. According to the method of the invention, hydrogen and oxygen can be separated out in different spaces and time, so that high-purity hydrogen is prepared; meanwhile, the electrolysis voltage is reduced, and the hydrogen production volume can be adjusted by controlling the amount of the electrolyte.