The invention relates to nanometer titanium dioxide having high uniformity and a small size, a nanometer titanium dioxide dispersion, and a preparing method and applications of the nanometer titaniumdioxide. The method includes S1) dissolving titanium tetrachloride into an ice-water mixture, and stirring the mixture to obtain a clear light-yellow liquid; S2) adding a surfactant into the clear liquid obtained by the step S1), fully stirring the mixture to fully dissolve the surfactant, adding dropwise aqueous ammonia in a speed of 40-80 [mu]L/s until the liquid is fully cured, then adding water into the liquid, stirring the mixture into a white turbid liquid system, and then adding dropwise aqueous ammonia until the pH of the system is 6-8; S3) allowing the system obtained in the step S2)to stand and be layered to obtain white precipitate, and washing, centrifuging, drying and grinding the precipitate to obtain white powder; and S4) calcining the white powder obtained in the step S3)at 350-800 DEG C to obtain the nanometer titanium dioxide. The nanometer titanium dioxide prepared by the method has a size of 10-40 nm, good dispersibility and stability, and excellent photocatalyticperformance. The method has few process steps, low equipment requirements and less site limitation, and is simple to operate and low in cost. The conversion rate of raw materials is high.