The invention discloses a Ta-Nb composite nickel-based welding wire for nuclear power equipment and a welding method thereof, relates to the technical field of welding materials, and solves the problem that existing welding wires are prone to crack defects under the severe condition of high restraint degree welding. The components of the welding wire consist of C, Si, Mn, S, P, Cr, Mo, Cu, Nb, Ta,Ti, Al, Fe, Ca, Mg, O, N, B, Zr, impurity elements and Ni. The welding method adopts automatic wire filling GTAW (Gas Tungsten Arc Welding). According to the Ta-Nb composite nickel-based welding wirefor the nuclear power equipment and the welding method thereof, when the surfacing thickness of the nickel-based welding wire reaches 30 millimeters, deposited metal still has no crack defect; when the nickel-based welding wire is used for welding, the process parameter adaptability is strong, an arc in a welding process is stable, the wire feeding performance of the welding wire is good, no dross exists, the forming performance is good, and the process performance is good; and micro-alloying of a welding seam is realized, and the welding seam meeting standard requirements is obtained. The Ta-Nb composite nickel-based welding wire for the nuclear power equipment and the welding method thereof are suitable for welding nuclear island main equipment.