The invention belongs to the technical field of water electrolysis and particularly relates to a three-electrode system double-electrolytic bath two-step water-electrolytic hydrogen producing device and method. The electrolysis device comprises two independent electrolytic baths 1 and 2, a hydrogen evolution catalyzing electrode, an oxygen evolution catalyzing electrode, a nickel hydroxide electrode and an alkaline electrolyte. According to the electrolysis device, the water electrolysis process is divided into two steps of hydrogen production and oxygen production which are respectively and alternately carried out in the two electrolytic baths 1 and 2. The device has the advantages that the steps of hydrogen production and oxygen production which are simultaneously generated in conventional water electrolysis can be effectively separated and respectively completed in different electrolytic baths, so that high-purity hydrogen can be produced under the condition of adopting no diaphragm and the water-electrolytic hydrogen producing cost is further reduced.