TiO2 modified steel slag-based permeable concrete (TSSPC) was prepared with nano-TiO2 incorporated as photocatalyst, steel slag and slag powder as binder, and large steel slag particle as aggregate. The effects of TiO2 content on the compressive strength, water permeability and NOx degradation performance of TSSPC were studied. In addition, combined with microscopic test and analysis methods such as XRD and SEM-EDX, the effect mechanism of TiO2 on the mechanical properties, water permeability and photocatalytic performance of TSSPC was investigated, laying the foundation for the application of TiO2 in permeable concrete. The results show that when the TiO2 content is low, the “nucleation effect” of TiO2 particles promotes the early hydration of binder materials. However, when TiO2 content is high, TiO2 particles tend to agglomerate, which is harmful to the mechanical properties of TSSPC. In addition, as the TiO2 content increases, the connected porosity and water permeability coefficient of permeable concrete increase. Compared with the blank control group, the connected porosity and water permeability coefficient of TSSPC with a TiO2 content of 5% (mass fraction, the same below) increase by 4.5 percentage points and 1.34 mm·s-1, respectively. The NOx degradation efficiency of TSSPC increases as the TiO2 content (0%～5%) increases. However, due to the agglomeration phenomenon, when the TiO2 content exceeds 3%, the effect of increasing the TiO2 content on NOx degradation efficiency decreases significantly.