The acid resistance of ultramarine blue was greatly enhanced by silica coatings. Sodium metasilicate nonahydrate was used as silica precursor and ammonium chloride was used to react with it. Fourier transform infrared (FT-IR) spectroscopy was used to characterize surface structure on the coating layer. The effects of coating conditions on acid resistances of the coated ultramarine blue were investigated. The acid resistance was evaluated by immersing the pigment in 10 wt% hydrochloric acid. The results showed the optimal mass ratio of silicate to ultramarine was 3.75 and the proper solid content was 60 g/L. Under above conditions, the silica coated ultramarine blue exhibited an acid resistance of 8, while the uncoated ultramarine blue only showed an acid resistance of 1.