The ammonia leaching residue sample was dissolved by HNO3-H2SO4-H3PO4 mixed acid and separated by NaOH. With Cu2+ and 2 mol/L H2SO4, Mo(VI) was reduced to Mo(V) by thiourea, then reacted with thiocyanate to form amber complexes which was used for the determination of Mo by spectrophotometric method. The sample digestion method, co-existence elements interference and their elimination were studied. The 8-time experimental results of the same samples by the proposed method were compared with that of gravimetric method and full-differential spectrophotometric method. The results indicate that the proposed method is simple, accurate and high-precision, and fit to the determination of Mo in ammonia leaching residue with satisfactory effects, prior to gravimetric method.