The composite photocatalytic material TiO₂ -CeO₂   was incorporated to steel slag asphalt mixture in tunnel environment in order to study its efficiency for exhaust gas degradation． Firstly，a complete set of tail gas degrada- tion test equipment capable of simulating tunnel environment was developed． Then the influence of composite pho- tocatalytic material TiO₂ -CeO₂    on the performance of steel slag asphalt mixture in asphalt pavement was studied． Finally，the optimum doping ratio of TiO₂ -CeO₂   composite photocatalytic material was also studied． The results show that when the nano-TiO₂ volume is 50% and nano-CeO₂ is 0. 7%，the photocatalytic efficiency will reach to the peak value． In this case，the degradation efficiency peak of NO and HC is 89% and 95%，respectively，and exhaust degradation efficiency reach up to 626. 3 mg/ (m³ ·min) and 74. 75 mg/ (m³ ·min) within 30 min，re- spectively． The road performance of tunnel steel slag asphalt mixture meets the requirements．