Secondary organic aerosol (SOA), a dominant air pollutant in many countries, threatens the lives of millions of people. Extensive efforts have been invested in studying the formation mechanisms and influence factors of SOA. As promising materials in eliminating air pollutants, the role of photocatalytic materials in SOA formation is unclear. In this study, TiO2 was employed to explore its impact on SOA formation during the photooxidation of m-xylene with NO x in a smog chamber. We found that the presence of TiO2 strongly suppressed SOA formation. The yields of SOA in the photooxidation experiments of m-xylene with NO x were 0.3-4%, whereas negligible SOA was formed when TiO2 was added. When ((NH4)2SO4) was introduced as seed particles, the presence of TiO2 decreased the yields of SOA from 0.3-6% to 0.3-1.6%. The sharply decreased concentrations of reactive carbonyl compounds were the direct cause of the suppression effect of TiO2 on SOA formation. However, the suppression effect was influenced by the addition of seed particles and the initial concentration of NO x. Reaction mechanisms of the photocatalysis of m-xylene with and without NO x were proposed.