The invention discloses car-washing water wax and a preparation method thereof. The car-washing water wax comprises the following compositions: 2-50 parts by mass of silane-modified diatomite, 2-20 parts by mass of a glazing agent, 2-15 parts by mass of a surfactant, 0.01-0.5 part by mass of essence, and the balance a solvent. The provided car-washing water wax can perform decontamination waxing on a car surface without water and machinery equipment, and helps to save water source and car owner time. Also, the car-washing water wax employs modified diatomite, guarantees cleaning effect, also prevents diatomite from wearing car paint, also forms a wax protective membrane on the car paint surface, reduces damage of dust storm, sunlight, acid rain and the like on car paint, and delays ageing of car paint. Also, the formula employs nontoxic free-stimulation compositions and does not generate any damage on car paint and construction personnel. The provided car-washing water wax is widely applicable to cleaning and maintenance of surfaces of automobiles, trains, machinery equipment and the like.