In order to solve the problem of formaldehyde (HCHO) volatilizing into the air and polluting the environment, a new type of highly efficient adsorbent was studied, and density functional theory (DFT) was used to calculate the influence of boron nitride (BN) nanocages of different sizes on the adsorption performance of HCHO. Firstly, BN nanocages of different sizes, namely B₁₂N₁₂、B₁₆N₁₆, and B₂₄N₂₄, were constructed. The HOMO and LUMO energy levels of the three nanocages were analyzed and compared, and it was found that B₁₂N₁₂ had the minimum energy level difference with HCHO. Therefore, charge transfer between B₁₂N₁₂ and HCHO was more likely to occur. In addition, the bond length formed by B₁₂N₁₂ and HCHO was the shortest (1.663 Å), and the absolute value of adsorption energy was the largest (0.653 eV), indicating that B₁₂N₁₂ had stronger adsorption and activation capacity for HCHO. Finally, by calculating the density of states,it is found that there is a more obvious orbital hybridization between B₁₂N₁₂ and HCHO, and the charge transfer is the highest, reaching 0. 332 e^-, indicating that the strong interaction between them makes the adsorption effect more significant. In summary，B₁₂N₁₂ with a smaller size shows great potential in formaldehyde removal.