Abstract: By applying EHMOmethod,a series of mimic calculations have been performed for the surface acid of Silica-Alumina catalyst,the basic states of Bronsted acid and the influence of Silica-Alumina ratio on Bronsted acidity have been studied.The calculations from different states of Bronsted acid show that the content of Aluminium in catalyst have a great influence upon the Dronsted acidity,and the forming of Al-O-Al structure is the essential reason for marked decrease of Bronsted acid strength.The results from theoretical calculations are in accordance with the experimental results of measurements of Bronsted acid.