The invention discloses a method for electrolytically extracting rare earth through liquid cathode molten salt and preparing lead rare earth alloy, and belongs to the technical field of nuclear fuel reprocessing. According to the technical scheme, lithium chloride, potassium chloride, and rare earth chloride are put into an alumina crucible, a mixture is subjected to temperature rise and is meltedto molten salt, a small crucible containing a lead ingot is put into the alumina crucible, and heat preservation is carried out; a cathode, a reference electrode and an auxiliary electrode are inserted into the molten salt to be connected with an electrochemical workstation; electrochemical workstation cyclic voltamogram and timing potential are used for measuring rare-earth reduction peak potential and alloy forming current; according to the reduction peak potential of the rare earth on the liquid-state lead electrode and the forming current of the lead rare earth alloy, constant potential electrolysis and constant current electrolysis are carried out for extracting the rare earth; the small crucible is cooled under the argon protection, an electrolysate is taken out, ethyl alcohol and deionized water are used for flushing, low-temperature drying is carried out, and the lead rare earth alloy is obtained. Compared with a method for extruding the rare earth through a solid electrode and obtaining the lead rare earth alloy, the method is short in flow process, and high in extraction rate, and the rare earth extraction rate is up to 97.2%.