. The Ce3+/TiO2 complex membrane made from Ce3+on the TiO2 framework with the activity of photocatalysis were prepared. The degradation behavior of the best mass ratio of Ce3+/TiO2 complex membrane , and the influences of the diameters of particles of Ce3+/TiO2 complex membrane on the degradation of the harmful gases under 254nm and 365nm ultraviolet light or natural light in room were investigated. AFM result showed that the Ce3+/TiO2 complex membrane were composed of nanoparticles with 10nm~50nm diameters of particles. The experimental results revealed that the smaller the nanoparticles in Ce3+/TiO2 complex membrane, the greater the degradation degree was. The best mole ratio of Ce3+ and TiO2 was 5% and the best mass ratio of Ce3+/TiO2 and formaldehyde were 7.06 under 254nm and 365nm ultraviolet and 23.55 under the natural light in room after 24h when the degradation percents of formaldehyde were more than 90%. Same as above conditions ammonia were 8.76 and 25.79 and benzene were 15.65 and 35.32. The degradation percents of Ce3+/TiO2 complex membrane to harmful gases were higher about 3 times under 254nm and 365nm ultraviolet than under the natural light in room.