Aiming at the technological shortage of national standard method for the determination of total phosphorus, a rapid determination method of total phosphorus based on ultrasonic assisted sample digestion-spectrum analysis was put forward, and the on-line analysis experiment system was designed. Relying on the experiment system, the experimental method and technology was studied. In view of the actual environmental water samples, contrast test experiment with the national standard method for determination method of total phosphorus was carried on. The experimental results showed that the digestion rate of ultrasonic assisted sample digestion using fenton reagents could reach 97%-100% in 13.5 minutes, and the determination method could complete one copy of sample analysis within 16 minutes. This determination method can solve national standard method's technical problems. All of this research work provides the important experimental basis and technical support for the development of the rapid on-line monitoring instrument for determination of total phosphorus.