The invention relates to a method for preparing a lead bismuth alloy for a nuclear reactor. In the method, the lead bismuth alloy is smelted by vacuum induction. The method comprises the following steps of: 1) putting lead serving as a raw material into a crucible in a glove box, heating to the temperature of between 450 and 800 DEG C, and introducing high-purity hydrogen for pretreatment; 2) passivating the crucible; 3) charging according to design ingredients, cooling the lead, putting into the crucible, and putting bismuth into a hopper; 4) introducing mixed gas of argon and the hydrogen under vacuum of between 0.1 and 10Pa until the mixed gas has the pressure of between 0.01 and 0.5Mpa, and melting at the temperature of between 400 and 600 DEG C, wherein the content of the hydrogen isbetween 2 and 7 volume percent; 5) adding the bismuth serving as a raw material in batch, and stirring in an electromagnetic and mechanical stirring combination mode; 6) detecting alloy ingredients by using a direct-reading spectrometer, and supplementing alloy elements according to a test result; and 7) pouring at the temperature of between 200 and 400 DEG C under the vacuum after the ingredients are qualified. By the method, the ingredients and purity of the produced lead bismuth alloy meet design requirements, and ingredient segregation and tissue segregation are prevented; the method is suitable for the nuclear reactor; and a process has the characteristics of short production time, high repeatability and low production cost and is easy to operate.