Silver-zinc slag, containing a lot of Bi, Ag, Cu, Zn, Pb and a little Au, has high comprehensive utilization value. This paper describes a preparation method of high-purity nanometer bismuth trioxide (Bi2O3) from silver-zinc slag by hydrometallurgical method. The results showed that Bi could be effectively leached out from silver-zinc slag using sodium chlorate, hydrochloric acid and sodium chloride as leaching agents, and the leaching rate of Bi was up to 99.03% under the optimum conditions. The contents of Pb2+ and Ag+ in BiCl3 solution could be reduced to about 1mg/L by using silver-zinc slag as reductant and sodium iodide (NaI) as precipitant, respectively. Crystalline form of Bi2O3 is affected greatly by concentration of NaOH, conversion temperature and time. The suitable temperature of BiOCl converting to Bi2O3 was 90 , and the suitable surfactant was polyvinyl alcohol. The average particle size of Bi2O3 obtained was about 86 nm, the purity of Bi2O3 was up to 99.75%, and the yield was 98.21%.