The purification technology and material of indoor formaldehyde is a hot spot in the field of material at present. In this paper, MnSO4, KMnO4, NaOH and H2O2 etc. are used to make manganese dioxide nanocrystal with the method of liquid precipitation. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to characterize the structure and properties. The results show that the products are nano-crystals of γ- MnO2 and δ-MnO2. The catalyze oxidation of formaldehyde using bothγ- MnO2 and δ-MnO2 as catalysts was investigated; and acetyl acetone method was used to characterize formaldehyde chroma. The catalytic ability ofγ-MnO2 nanocrystal to catalyze formaldehyde oxidation is better than that ofδ-MnO2.