Two-dimensional (2D) Janus transitional-metal dichalcogenides (TMDCs) have great potential for photocatalytic water splitting due to their novel properties induced by the unique out-of-plane asymmetric structures. Here, we systematically investigate the geometric, electronic, and optical properties of 2D Janus MoSSe with titanium doping and vacancies to explore their synergistic effects on photocatalytic activity. We find that there is effective attraction between the substituted or adsorbed Ti atoms and S/Se vacancies. The Ti adatoms dramatically extend the light absorption range to the infrared region. The S/Se vacancies coexisting with Ti adatoms will modulate the transition of photoexcited electrons, thereby enhancing sunlight absorption. The Ti adatoms either existing alone or coexisting with vacancies introduce smaller lattice distortion compared with substituted Ti atoms, and these Ti adatoms induce smaller effective mass of charge carriers. The configuration of S vacancy coexisting with Ti adatoms on the Se surface exhibits the most significant synergistic effects and the best overall photocatalytic performance. Our work reveals the mechanisms and effects induced by doping and vacancies coexisting in 2D Janus TMDCs and also proposes a new practical strategy to improve the performance of 2D photocatalysts.