Abstract: The single affecting factor method is combined with the orthogonal test to study the interference of arsenic speciation and the main interference factor, and to determine the optimal amount of sodium thiosulfate and the best developing time in the ammonium molybdate spectrophotometric detection of total phosphorus in water. Results showed that the interference effects of arsenic species are: arsenate > arsenite >> dimethylarsinic. When the arsenic concentration is lower than 2.0 mg/L, the developing time is the main influence factor. When the arsenic concentration was in the range of 0.1~5.0 mg/L, the optimal amount of sodium thiosulfate (40 mg/mL) was 0.5 mL and the best developing time was 15 min with the consideration of the practical operation and real sample.