The invention discloses a preparation method of brazing filler metal for lead-free tin-silver-copper brazing. The preparation method comprises the following steps that firstly, raw materials Sn, Ag, Cu, Ge, Mn, Ni and a P-Cu intermediate alloy are weighed and taken to enable that the Ni element accounts for 1%-2% of the total mass, the Ag element accounts for 2.0%-3.0% of the total mass, the Cu element accounts for 1.5%-2.5% of the total mass, the P element accounts for 0.5%-1.5% of the total mass, the Ge element accounts for 1%-2% of the total mass, the Mn element accounts for 1%-2% of the total mass, and the balance is the Sn element; the weighed raw materials are uniformly mixed, smelt in a heating furnace at the temperature of 1000 DEG C-1200 DEG C, and cast into an ingot; oxide skin and dead heads of the cast ingot are removed, and the cast ingot is made into a wire by adopting a hot extrusion forming method; and after vacuum annealing is carried out, the wire is processed into blocky brazing filler metal, rod-shaped brazing filler metal, sheet-shaped brazing filler metal or filiform brazing filler metal according to use requirements. According to the lead-free tin-silver-copper brazing filler metal prepared through the method, under the condition that good thermal performance is guaranteed, the wettability and the oxidation resistance are further improved, and the higher requirement of electronic products for the performance of welding spots is met.