For its low concentration indoor, formaldehyde is hard to degradation. The mechanism of nano-TiO2 photocatalysis degrading indoor formaldehyde gas is explored. To improve photo catalytic degradation rate of formaldehyde, the modification of TiO2 catalyst, the adoption of strong absorptive catalyst support, the improvement of avability ratio of light, the ultrasonic assisted catalytic and the combination with pulsed discharge plasma were discussed in detail as the proposed methods. The future tendency of degradation indoor formaldehyde by TiO2 in visible light was predicted.