The invention discloses a preparation method of coal-based activated carbon with low ash and high specific surface area. The preparation method comprises steps as follows: firstly, coal is crushed to be smaller than 150 mu m, pulverized coal is obtained, a strong alkali compound is added, the mixture is mixed uniformly and is heated and activated under the protection of inertial gas, and an activated product is obtained; after the activated product is subjected to acid pickling and washing until a filtrate is neutral, the strong alkali compound is added, the mixture is mixed uniformly and heated to a constant temperature under the protection of inertial gas, then the inertial gas is switched to vapor for activation, and a final activated product is obtained; finally, after the final activated product is subjected to acid pickling and washing until a filtrate is neutral, an activated carbon product with low ash and high specific surface area is obtained through drying. According to the preparation method, the coal-based activated carbon with low ash and high specific surface area is prepared by adopting anthracite, semianthracite, bitumite and the like as raw materials, and the activated carbon product prepared with the method has high specific surface area, large pore volume and low ash content.