Synthesis of large scale ultrathin 2D structural TiO2 is challenging and meaningful in many fields of science and technology, because of their larger surface area and higher electron-hole pairs separation efficiency. In this work, a "bottom-up" method is used to synthesize the large-size ultrathin TiO2 nanosheets at low temperature by liquid-phase. The mixture of tetrabutyl titanate and ethanol could hydrolyze in a dilute nitric acid solution with an ice-water bath to result the hydrolysates that could peptize. After peptized, the hydrolysis products become to be very small TiO2 nanoclusters and they form a two-dimensional network structure via the orientation bonding formed by hydrogen bond. Continue to be aged at low temperature, the crystallization degree of samples will increase, and the networks eventually turn into TiO2 nanosheets. Effects of the concentration of nitric acid, ambient temperature and reactant concentration on the formation of two-dimensional structure TiO2 are studied in this work. The transmission electron microscope (TEM), ultraviolet-visible (UV-Vis) absorbance spectra, X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy (FTIR) are used to analyze the morphology, microstructure and properties of the samples, and the experiment of photocatalytic reduction of Cr(VI) is conducted to observe the photocatalytic activity of samples as well as to verify the effects of system parameters on the microstructure of TiO2 nanosheets. The results show that when the concentration of nitric acid is during 0.0217~0.0721 mol·L-1, the anatase TiO2 nanosheets with obvious quantum size effect that thinner than 1 nm could be obtained by peptizing and aging at 0~4 oC. Excess nitric acid leads to crystalline and morphology transformation of TiO2, but lower concentration of nitric acid could prolong the peptizing time; increasing the ambient temperature will undermine the formation of two-dimensional structure; improving the amount of ethanol in the reactant will be helpful to disperse the hydrolysis products, and promote the process of the peptizing and formation of the two-dimensional structure.