Abstract: Portable spectrophotometric method was used to determine total phosphorus in water. The effects on the experimental results of digestion temperature, digestion time, color temperature, color time, interference factors and the feasibility of self-compounding reagents replacing prefabricated reagents were studied. At the same time, three domestic mainstream brands of instruments and equipment were selected to carry out comparative experiments to determine the optimal condition. The optimal conditions were determined as follows: 125±5℃ of digestion temperature,30 min of digestion time, 15 ～ 40℃ of color temperature, and 15 min color time. The interference can be removed by compensation of chroma and turbidity. Self-compounding reagents were a good substitute for prefabricated reagents.By measuring standard samples and actual samples, the detection limit of the method was 0.05 mg/L, the relative standard deviation was between 1.2% ～ 11.8%, recovery rate was between 80.0% ～ 120%. It is especially suitable for the emergency monitoring of the scene of sudden pollution accidents, which can grasp the water quality status in time and provide data support for environmental management decisions.