The invention provides a binary composite La2O3 and Ta2O5-doped molybdenum negative electrode material and a preparation method thereof, and belongs to the technical field of rare-earth refractory metal negative electrode materials. The total additive amount of active materials La2O3 and Ta2O5 in a negative electrode substrate is 3-5wt%, wherein the additive amount of the Ta2O5 is 0.05%-0.5%; and the balance is Mo. The method comprises the steps of mixing a lanthanum nitrate solution, an ammonium heptamolybdate solution and a citric acid solution, adding tantalum oxide powder and then carrying out water-bath heating, mechanical stirring and drying to finally form dried gel; decomposing the dried gel to obtain La2O3 and Ta2O5-doped MoO3 powder; doping the La2O3 and the Ta2O5 obtained from the mixed oxide powder through a secondary reduction process into molybdenum powder; carrying out pressing, sintering, carbonizing, activating and seasoning treatment on the powder to obtain the negative electrode material with good emission property. The material is excellent in thermionic emission property and meets the operating requirements of a negative electrode in a microwave oven magnetron.