Ammonia slip catalyst (ASC) prevents the emission of ammonia to the atmosphere by oxidative decomposition of excess ammonia in the urea SCR system. In a series of studies aiming at the development of the plant model of ASC, we tried the method of making a mixed catalyst model from two types of catalyst models of SCR and PGM which constitutes ASC. The mixed catalyst model with NH3 adsorption sites on Cu-SCR and PGM was able to accurately simulate NH3 oxidation activity. We discussed the technical points of the new method of making a mixed catalyst model and applicable objects.