The development of small total phosphate testing system and the study of phosphate electrode are proposed in order to optimize the pretreatment system of small water sample digestion. Digestion is performed for each electrode before testing. A digestion pretreatment system for water treatment based on ultraviolet photo-catalytic method is optimized. A 254 nm wavelength UV light is used in this system, and gold is selected as sensing material. The volume of digested water sample or actual sample is measured. These electrodes are applied to detect total phosphorus concentration in water samples from lake. The experiment results demonstrate that the system shows high sensitivity in the concentration range of 0.1 mg/L–1.0 mg/L(R2＝0.993 3) and a low detection limit (0.1 mg/L) in the slope around 1.477 0 μA/(mg?L-1). The proposed total phosphate testing system bears the characteristics of small size, low power consumption, high digestion efficiency and has no need of strong oxidizing agent, which is expected to be used for online-monitoring of total phosphorus.