Abstract: Under the conditions of 0.2mol/L sulphuric acid -10% ethanol medium and irradiation of UV-light, molybdophosphate heteropoly acid is reduced to its heteropoly blue (λ_max=780nm) On those grounds,a new photochemical spectrophotometric method for 0.05～2.0μg/mL phosphorous has described, and applied to determining phosphorous in real samples The photochemical mechanism is also discussed.