The solid state reaction method, coprecipitation method and citric acid method were employed for the preparation of Cr-Mn mixed-oxide catalysts. Experimental results showed that this catalytic system has good selective catalytic reduction (SCR) properties of NOx by ammonia in the presence of excess oxygen at low temperature (80-220°C). 97.8% conversion of NOx with 100% selectivity of N2 had been achieved on Cr-MnOx mixed oxide prepared by the citric acid method at 120°C with the space velocity of 30,000 h-1. X-ray diffraction, BET were adopted for the characterization of the active phase. The characterization results showed that a new crystal phase CrMn1.5O4 generated in mixed oxides which was the active center of the Cr-Mn catalysts.