The invention discloses a preparation method of a microwave attenuation film for a klystron. According to the method, FeSiAl powder is used as main material, iron powder, nickel powder and copper powder are additives, a guncotton solution and diethyl oxalate are added into the FeSiAl powder to form paste, the paste coats the inner surface of an oxygen-free copper cavity and is sintered in hydrogen gas, and finally the microwave attenuation film is obtained. According to the preparation method of the microwave attenuation film for the klystron, the operability is high, the process design is reasonable, the weight percentage of the iron powder, the nickel powder and the copper powder and the weight ratio of Fe, Si and Al in the FeSiAl powder are screened out through a large number of experiments, the optimum sintering process is screened out through a large number of experiments, the whole process design is reasonable, and the operability is high. The prepared microwave attenuation film is not prone to fall off or crack, is good in high temperature resistance and excellent in attenuation performance and can obviously lower the Q value of a cavity.