The invention provides an electrolytic bath and a method for electrolyzing light rare earth metals or alloys. The wall of the electrolytic bath is composed of an incrustation protection layer 2 of an electrolyte solution 8 composed of rare earth fluoride-lithium fluoride, a starting cathode 5 is installed at the bottom part of the electrolytic bath, a plurality group of graphite anodes 1 are installed right above the starting cathode, each group of graphite anodes is composed of a graphite anode B to be electrolyzed at the upper side and a graphite anode A being electrolyzed at the lower side, and the graphite anode A is connected with the graphite anode B through threads 15. The contents of ferrum and carbon in the rare earth metals or alloys electrolyzed by the electrolytic bath are low; the generation of waste graphite anodes in the electrolytic process is quite small, even no waste graphite anode is generated. The electrolytic bath and the method for electrolyzing light rare earth metals or alloys provided by the invention are simple and feasible.