Abstract: A study on mechanism of compact silicon coating of titanium pigment was carried out.Through thermodynamic analysis of silicic acid aqueous solution system,the advantage existing zones of H₅SiO₄⁺, H₄SiO₄,H₃SiO₄^- and H₂SiO₄^2- produced by hydrolysis of silica gel were drawn.With the addition of silicate concentration,the advantage existences tend to move to higher pH.It can be concluded from the experiment of gel formation and thermodynamic analysis that the gel formation rate is determined by the concentration of both H₄SiO₄ and other silicate ions.When the distrabution coefficient of H₄SiO₄ and H₃SiO₄^- are equal,the product of their concentration is the largest at certain total concentration of silicate ions.With the increase of silicate ions' total concentration,the gel formation rate speeds up.If the rate is out of control,flocculent silicate would form,which affects the coating compactness.With the temperature of 80℃ and silicate ion' concentration of 0.2 mol/L,0.3 mol/L and 0.4 mol/L,the pH value of the system should be greater than 9,9.5 and 10 respectively.