The invention discloses a preparation method of a water permeable brick. The preparation method of the water permeable brick comprises the step of preparing a cementing material, wherein raw materials of the cementing material comprise silicon-calcium slag. According to the preparation method of the water permeable brick, firstly the silicon-calcium slag is subjected to thermal activation treatment, then the cementing material is prepared by taking thermally activated silicon-calcium slag as a main raw material, finally the silicon-calcium slag cementing material is taken as a binding agent and is mixed with stone aggregate, and stirring and moulding are carried out, thus the water permeable brick is prepared. The prepared silicon-calcium slag cementing material is an alkali-activated material and can react with the stone aggregate, and a formed final hydration product is C-S-A-alpha(Na, K and other alkali metal ions)-H system gel, so that the cementing material and the stone aggregate form a whole, a preparation process of the water permeable brick is firing-free and steaming-free, and the water permeable brick is moulded at normal temperature and has high compressive strength and good water permeable effect.