Abstract: Nano-Ce/SnO₂ photocatalyst was obtained by the chemical precipitation method, The particle crystal form, size of powders, morphology have been characterized using the X-ray diffraction and electron microscope.It was found that the prepared catalysts were quartet rutile structure and the average particle size were between 40 to 50 nm.Under visible light, the photocatalytic oxidation of ammonia nitrogen in seawater was researched by using prepared nano-SnO₂.The influence orders of degradation as follows:Ce-doping ratio >calcination temperature >initial concentration of ammonia-N >reaction time >photocatalyst dosage.According to the orthogonal experiment and the verified experiment, The optimum reaction conditions:photocatalyst dosage is 0.8 g/L, Ce-dopping ratioa is 1.0%, calcination temperature is 500℃, initial concentration of ammonia-N is 50 mg/L, reaction time is 2 h.Under such condition, the removal rate of ammonia-N can reach 92.7%.