The invention relates to a method for preparing boron carbide-silicon carbide composite ceramic in situ by using a mortar cutting waste material as a raw material. The method comprises the following steps of (1) crushing the mortar cutting waste material and carrying out acid leaching to remove impurities to obtain an impurity-free waste material; (2) grinding the impurity-free waste material until the particle size is lower than or equal to 60 microns to prepare impurity-free powder; (3) grinding a carbonaceous reducing agent until the particle size is lower than or equal to 60 microns to prepare carbonaceous reducing agent powder; (4) uniformly mixing the carbonaceous reducing agent powder, the impurity-free powder and boron carbide powder, adding water and a binder, and pressing the mixture into square lumps through a ball pressing machine; (5) drying the mixture at 50-120 DEG C to obtain raw blanks; (6) putting the raw blanks into a vacuum hot-press sintering furnace. The method obviously improves the uniformity of silicon carbide and the bonding condition of the interfaces of boron carbide and silicon carbide in the material, and enables the sintered boron carbide-silicon carbide composite ceramic to have better performance.