The photoexcitation dynamics plays a key role in determining the properties of van der Waals heterostructures (vdWHs). Based on the time-dependent density functional theory combined with nonadiabatic molecular dynamics, we investigate the charge transfer in Janus-MoSSe/WS2 vdWHs. Ultrafast charge separation is observed, arising from the large overlapping between the donor and acceptor states. While the electron-hole recombination is 2 orders of magnitude slower than the charge separation, this can be understood by the fact that the initial and final states are strictly confined to different materials. Additionally, photoresponsivity performance of the vdWHs is also evaluated using density functional theory combined with the nonequilibrium Green's functions. Simulated results of high photoresponsivity in a broad range of the spectrum endows proposed systems powerful potential in optoelectronic and photovoltaic applications. The atomistic picture revealed in our work provides chemical guidelines and facilitates the design of next-generation devices for light detecting and harvesting.