In order to evaluate the photocatalytic degradation capacity of self-prepared nano-sized TiO₂ for the organic pollutants in the air, a self-developed photocatalytic degradation reaction system was adopted to analyze the photocatalytic degradation process of formaldehyde and to investigate the factors influencing the degradation, such as the relative humidity, the illumination intensity, the gas flowrate and the initial mass concentration of formaldehyde. The results indicate that the photocatalytic degradation performs best at a relative humidity of 50% and a gas flowrate of 1.20 L/min; and that the photocatalytie degradation rate does not increase unlimitedly with the increase in illumination intensity, but it decreases with the increase in the initial mass concentration of formaldehyde.