Anatase TiO2 nanoparticles with the size of about 20 nm were synthesized using Ti(SO4)2 as titanium source and stronger ammonia water as precipitant at 240 °C for 48 h with pH=9 by a hydrothermal hydrolysis method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and ultraviolet-visible absorption spectroscopy (UV-VIS). XRD analysis shows that the phase of the sample is anatase TiO2. SEM and TEM analysis confirms that with the increase of the reaction time, the size of TiO2 nanoparticles prepared without stronger ammonia water can reach nanometer grade, but its change is not obvious. With the addition of stronger ammonia water, the size of TiO2 nanoparticles becomes small noticeabley. With the further increase of pH value, its influence on the TiO2 nanoparticles size is not obvious. And UV-VIS analysis shows that the reduction in the size of anatase TiO2 nanoparticles is beneficial to the blue shift of their absorption peak.