Abstract: In this article,by applying EHMOmethod a scries of mimic calculations have been performed for the surface acid of silica and silica-alumina catalyst formed by adding aluminium oxide in silica gel.We have studied the change of Lewis acid strength of mimic catalysts in which silicon have been substituted by aluminium in different position and number.The micromechanism of conversion between Lewis and Bronsted acid also has been studied.The calculations show thai the amount of aluminium in catalyst have a very little influence upon the strength of Lewis acid,even if Al-O-Al structure is formed the Lewis acid strcngth decreased only a little.Then we show theoretically the mechanism of conversion between Lewis acid and Bronsted acid and the nature of Bronsted acidity.Aseries of results from theoretical calculations are in accordace with the experimental results of measurements of Lewis acid and photoelectron spectroscopy.