Abstract: Phosphate ore sample was dissolved with nitric acid and chlorhydric acid. Metaphosphoric acid in solution was oxided into ortho-phosphoric acid. Reacting with ammonium molybdate under the catalyst of bismuth salt, phosphorus transferred yellow ammonium phosphomolybdate. The ammonium phosphomolybdate was reducted into phosphomolybdenum blue. Then the contents of phosphorus were determined with parallax photometric method. The maximum absorbance is at 700 nm with an apparent molar absorptive of 1.60×10⁵L/(mol·cm). Beer's law is obeyed in the phosphorus concentration range of 3～5 μg/mL. The recovery was in the range of 97.10%～100.86% while the relative standard deviation was in the range of 1.08%～4.66%. The method is suited to determine the phosphorus pentoxide contents with the range of 5%～38% in phosphate ores.