The invention discloses a preparing method of AuSn20 brazing filler metal for improving braze performance. A single roller melt-spinning quick set technology is adopted for preparing the AuSn20 foil brazing filler metal, and the preparing method comprises the following steps that (1) raw materials are prepared, wherein tin and gold raw materials with the purity being 99.99% are selected, and the raw materials are prepared according to the mass percentage being 1:4; (2) alloy smelting is carried out, wherein smelting is carried out in a vacuum induction furnace, the vacuum degree is 1*10<2>-1*10<-6> Mpa, repeated overturning smelting is carried out for 3-4 times, and a gold-tin alloy ingot is obtained; and (3) melt-spinning is carried out. Compared with a technical method for preparing the AuSn20 brazing filler metal, the preparing method has the beneficial effects that the alloy achieves complete alloying, the smelting point of the brazing filler metal is reduced, the welding performance is better, and the brazing filler metal foil thickness is controllable. In addition, according to the method, operation does not need to be carried out under high vacuum and the protective atmosphere, the process operation is simple, cost is low, and the preparing method is suitable for batched production.