The invention relates to a method for preparing a nickel base alloy welding wire. The nickel base alloy comprises, by weight, 0.4%-0.45% of C, 1.0%-1.5% of Mn, 0.15%-1.2% of Si, 0%-0.025% of P, 0%-0.02% of S, 28%-30% of Cr, 49%-51% of Ni, 0%-0.1% of Co, 0%-0.1% of Mo, 0%-0.1% of Ti, 0%-0.1% of Al and 4.0%-5.0% of W, and the balance iron. The preparing method comprises the step of alloy smelting, electroslag remelting, hot forging cogging, hot rolling annealing, acid pickling, coping, cold drawing and hydrogen retreating, and a CaF2, Al2O3 and CaO slag system is adopted in the electroslag remelting. According to the method for preparing the nickel base alloy welding wire, the thin welding wire with the performance meeting the requirement can be obtained, and the yield is high.