The invention relates to a preparing method of a nanometer calcium hydroxide powder material for fixing ancient murals. The preparing method comprises the steps of firstly dissolving calcium chlorideinto water, and afterwards, adding dimethyl formamide; mixing a calcium chloride aqueous solution with a sodium hydroxide aqueous solution to prepare nanometer calcium hydroxide. By means of the preparing method, heating is not needed, preparation is conducted at the normal temperature, and synthesized nanometer calcium hydroxide has high stability, and does not precipitate for 3 months in ethyl alcohol. The synthesis process is very simple, the time is saved, and it takes less than 1 h in the whole process. Besides, the method is low in synthesis cost, overcomes shortcomings of a current method of synthesizing nanometer calcium hydroxide, and has a very good industrial production prospect. As is shown by mural reinforcing experiments, the material is suitable for reinforcing the murals ofhumid burial chambers, and particularly well protects mural efflorescent and flaking of the murals. Compared with the prior art, the preparing process of the nanometer calcium hydroxide material is very simple, the time is saved, the preparation is controllable, and the cost is low. Besides, heating is not needed, preparation is conducted at the normal temperature, and energy consumption is low.