Abstract: The concentration of total phosphorus(P-tot) in effluent is an important indicator of the effect of wastewater treatment plant(WWTP), excessive discharge of phosphorus will result in the tender eutrophication of natural water and other ecological pollution problems. In this paper, Qilidian WWTP in Guilin for the study object, researched Carrousel oxidation ditch process of the plant DO, pH, nitrate, BOD5 and P-tot in influent, and P-tot in influent and in effluent of the final clarifier, systems analyzed the factors and reasons affecting phosphorus removal. The result of the research shows itself as follows: the dissolved oxygen (DO) in oxidation ditch anaerobic zone above 0.5mg/l and at the end of the aerobic zone up to 3.87mg/l are not suitable for phosphorus accumulating organisms (PAOs) to release phosphorus; in the influent, B0D5/P-tot<20, carbon source used for PAOs in the sewage is insufficient; the concentration of P-tot in the effluent is higher than that in the influent, there occurs an obvious phenomenon of phosphorus release in the final clarifier. In addition, the pH in oxidation ditch ranges from 6.82 to7.19 and the concentration of nitrate in anaerobic zone below 0.2mg/l are suitable for the phosphorus removal process. This research provide technical support for the similar oxidation ditch process of upgrading and transformation,and have a higher value in engineering practice.