The invention discloses a method of controlling the grain size of a precisely cast turbine blade. The method comprises the following steps: overlapping a disturbing rib to the exhaust edge of a casting wax mold, coating the surface layer with a surface sizing agent, then coating the surface layer with a back coating as a reinforced layer, finally dewaxing and roasting, taking the residual wax out, then putting a casting shell into a heat preservation barrel, heating and casting. The operation steps of the method are simple, the aim of controlling the grain size of the blade is realized by overlapping the disturbing rib in a wax mold combining stage, extra cost or energy consumption is avoided, and the operation is convenient, safe and pollution-free; the heat preservation barrel which is as high as the casting shell is manufactured by a high-temperature alloy panel, the shell is put in the heat preservation barrel to be roasted, the temperature drops of the shell in the drawing and vacuumizing processes are reduced due to the heat preservation function of the barrel which absorbs heat and prevents direct convective heat transfer between the shell and air, temperature difference between the casting shell and the casting temperature is ensured, and improvement on the grain size of the casting is benefited.