The improvement of spectrophotometric determination of organic phosphorus pesticide by phospho-molybdenum blue was studied.The effects of wave-length,chromogenic reaction and coexisting ions were investigated.The sensitivity,accuracy,precision and stability of the improved method were compared with GB/T 6913-2008.A good linear relationship between absorbance(A) and phosphate mass(μg) was observed under the condition of : wavelength 890 nm,2.00 mL ammonium molybdate solution(26 g/L),1.00 mL ascorbic acid(100g/L,containing 0.4 g /L EDTA),and incubated at 45℃ for 10min,.The equation was expressed as Y=0.004 53X-0.002 29(R2=0.999 84),and the apparent molar absorption was 2.118×104 L/(mol·cm).The mass concentration of PO3-4 is 0~160 μg and fits to Beer's law.Mean recoveries of phosphate varied from 95.199% to 102.097%.The method is easy to operate and applicable to the requirements of organophosphorus pesticide residues detection.