The invention belongs to the technical field of preparation of nanoscale lithium salt and particularly discloses a method for preparing high-purity lithium carbonate. The method includes the steps that firstly, pure water is heated to 60-80 DEG C, the pure water and lithium hydroxide monohydrate are mixed with the mass ratio being two to one, then, a small amount of water-soluble dispersant is added into the mixture and is sufficiently stirred, and the mixture serves as a lithium source; secondly, percarbamide and pure water are mixed in equal mass to serve as a carbonic acid source, and the molar ratio of the percarbamide to the lithium hydroxide monohydrate is 1:1.80-1.95; thirdly, the lithium source and the carbonic acid source are added into a hydro-thermal synthesis reaction still and are sufficiently stirred, then, the still is closed and placed in a drying oven, the still is heated to 100 DEG C-120 DEG C at the speed of 3 DEG C/min and is taken out after being contained for 5-10 h, lithium carbonate size in the still is filtered to obtain a filter cake, the hot filter cake is washed for 2-4 times through boiling pure, and then wet materials are obtained; fourthly, the wet materials are dried for 8-15 h at the temperature being 250 DEG C-350 DEG C, the pressure being -0.1 Mpa- -0.08 Mpa, and finally lithium carbonate powder with the particle diameter being 20-90 nm is obtained.