The invention discloses a hydrofluoric acid modified waste brick, a composite based on the waste brick and application of the composite to adsorption of heavy metals and purification of water. A preparation method of the hydrofluoric acid modified waste brick comprises the following steps: S1, crushing the waste brick, sieving and grading, and selecting brick particles with size of 0.5-1.2mm; S2, washing the brick particles with water, filtering the brick particles, and drying the brick particles at the temperature of 100-110 DEG C; step S3, putting the brick particles obtained in the step S2 in a hydrofluoric acid solution for carrying out ultrasonic immersion modification, wherein the molar concentration of the hydrofluoric acid solution is 0.5-10mol/L, and the volume of the hydrofluoric acid solution is 15-25 times of the mass of the brick particles; and step S4, washing the modified brick particles until the brick particles are neutral, and drying the brick particles at the temperature of 100-110 DEG C. A preparation method of the composite based on the modified waste brick comprises the following steps: soaking the modified waste brick in titanium dioxide sol, taking the waste brick out, then drying the waste brick to obtain an amorphous titanium dioxide loaded modified waste brick. The composite is prepared by loading the amorphous titanium dioxide on the modified waste brick; the adsorption capacity to heavy metals in the water of the composite is obviously higher than the sum of the separate adsorption capacities to heavy metals in the water of the modified waste brick and the amorphous titanium dioxide.