The invention relates to a preparation method of formaldehyde trapping gel. The method comprises the steps as follows: octadecylamine and carbon tetrachloride are mixed in proportion and stirred uniformly to form a system; nano lead oxide, nano iron oxide, nano calcium carbonate, nano titanium dioxide nitride and sodium metabisulfite are mixed in water in proportion and uniformly stirred to form amixed solution; the mixed solution is slowly dropwise added to the system at the constant speed, a dispersant is slowly added, the mixture is stirred at 50 DEG C for 2 h, and a transparent sol mixture is obtained; the mixture is heated to 80 DEG C to be stirred continuously for 4 h, filtration, separation, extraction and drying are performed, and the formaldehyde trapping gel is obtained. Uniformmixing of the main formaldehyde adsorbing substances octadecylamine and nano titanium dioxide nitride in the molecular sense is realized by the aid of a sol-gel process, and the adsorption capacity of the finally formed trapping gel is strengthened.