The CFD model validated by wind tunnel experiments was used to study the impacts of green walls and thermal effects on diffusion of pollutants and the NO_x-O₃ photochemistry within the shallow street canyons. Four wall heating scenarios and green walls with four leaf area densities were considered, and ozone depletion rate was used to measure the severity of NO_x-O₃ photochemistry. Results show that wall heating can change the flow structure within the street canyon. Leeward wall heating and ground heating can increase the size of clockwise vortex within the canyon, the windward wall heating can weaken the vortex size, and whole wall heating can destroy the vortex structure. However, the setting of green walls can promote the vortex structure reconstruction, effectively reduce the average air temperature in the canyon, and reduce the average ozone depletion rate. Therefore, the green walls plays a positive role in alleviating urban heat island effect and photochemical pollution.