Abstract: Molybdenum ion in impregnation solution of Co-Mo catalysts was determined based on reaction of phosphorus molybdenum blue. A system for measuring high molybdenum ion concentration was established based on the spectrophotometry of flow injection-phosphorus molybdenum blue. The optimized conditions of the system found were as follows:① acidity regulator:0.5 mol·L-1 H2SO4 solution;② carrier:ultrapure water;③ reaction reagent:mixture of 120 g·L-1 KH2PO4,100 g·L-1 Vc and 0.3 mol·L-1 H2SO4;④ total flow rate:2.8 mL·min-1;⑤ length of acidity adjusted coil:20 cm;⑥ length of reaction coil:250 cm. The linear relationship between values of the peak height and the mass concentration of MoO3 was obtained in the range of 10.0-300.0 g·L-1,with detection limit (3s/k) of 2.0 g·L-1. The proposed method was applied to the analysis of samples of impregnation solution of Co-Mo catalysts,giving recovery rates in the range of 99.7%-105% with RSD′s (n=11) less than 1%.