Xylene is one of the most important anthropogenic volatile organic compounds (VOCs) for the secondary organic aerosol (SOA) formation. As the effect of NH3 on the yield and reaction mechanism of SOA formed by photooxidation of xylene is not clear, SOA formation from atmospheric photooxidation of xylene with different NH3 concentrations was studied in a laboratory smog chamber, focusing on the mass concentration, optical properties and chemical composition of SOA. The results show that NH3 can obviously promote the formation of SOA at low concentration. And according to aerosol mass spectra results, the promotion of carbonyl compounds into the particle-phase and the formation of nitrogen-containing organic in the presence of NH3 are the main reasons for the increase of SOA mass concentration. In addition, it is found that NH3 can increase the mass absorption coefficient (MAC) of o-xylene SOA, but has no significant effect on p-xylene SOA. The analysis shows that the formation of aldehydes through photooxidation of o-xylene is more efficient than that of p-xylene, and then Maillard reaction between NH3 and aldehydes leads to the increase of light absorption of SOA.