The influence of synthesis condition on pure nano-TiO_2 powder is reviewed.The advances in synthesis of ion-doped TiO_2 powder, coupled semiconductor powder, rare metal deposited TiO_2 powder and solid super acid TiO_2 by sol-gel method are summarized. The mechanisms for the enhancement of the photocatalytic activity of the modified nano-TiO_2 are introduced and enhancement mechanism lies in the separation of electron-hole pairs or the variation of electronic band structure or surface state.As to the proble...