In this study, a new type of photocatalytic composite material “sulfur-doped carbon nitride/calcined magnesium aluminum hydrotalcite (S-g-C3N4/MgAl-CLDH)” was synthesized and loaded on cement mortar through internal doping. The nitrogen oxide (NOx) degradation and hydration performance of photocatalytic mortar were tested. The results show that when the photocatalyst dosage is less than 5% (mass fraction), the S-g-C3N4/MgAl-CLDH content is directly proportional to the NOx degradation rate of mortar. When the photocatalyst dosage exceeds 5%, the NOx degradation performance of mortar begins to decrease due to agglomeration. In addition, due to the nucleation effect of nanomaterials, the incorporation of an appropriate amount of S-g-C3N4/MgAl-CLDH significantly improves the early (1 d) hydration degree of cement. However, due to its water absorption and agglomeration phenomenon, excess S-g-C3N4/MgAl-CLDH reduces the later hydration degree of cement. When the photocatalyst dosage is 7%, the 28 d strength of cement mortar significantly decreases compared with the control mortar. In addition, an appropriate amount of S-g-C3N4/MgAl-CLDH (3%) can improve 28 d strength of mortar due to its micro-filling effect.