La-doped nano TiO2 colorless and transparent hydrosol with an average particle size of 6.9 nm was synthesized by a novel complexation-controlled hydrolysis method at room temperature by using TiCl4 , organic carboxylic acid, stabilizer, surfactant, NH3 ·H2 O, La(NO3 )3 ·6H2 O, and D-sorbitol etc as raw materials. And the acid red 3R was studied by the photocatalytic degradation. To study the effects of reflux time, pH and concentration and reaction conditions of the synthesis process and properties of the materials, the optimum preparation conditions were found out. The samples were characterized by X-ray diffraction (XRD), UV-visible spectroscopy (UV-Vis), and nano particle size analyzer. The La-doped nano-TiO2 transparent photocatalyst exhibits optimal photocatalytic performance when the doping content was 0.1% and the refluxing time was 10 min, respectively. The degradation efficiency of acid red 3R dye solution with concentration 25 mg/L has reached 98% after being exposed to sunlight for 60 min.