It is important to determine the time-dependent evolution of the excited monolayer WS₂, which will provide a basis for the reasonable design of optoelectronic devices based on two-dimensional transition metal dichalcogenides. Here, we made a simple and large-area photodetector based on the monolayer WS₂, with high light sensitivity and fast response, benefiting from the special dynamics of carrier involving the exciton, trion, and charge. Moreover, we tested the relaxation behavior of the excited monolayer WS₂ by employing transient absorption (TA). It was found that the multi-body interaction among exciton would occur after the density of pump photon increases to 3.45 × 10¹⁴ photons per cm². The exciton dissociation accompanying the generation of trion would appear in the photo-induced relaxation process, which would be a benefit for the operation of this photodetector. Increasing the energy of the exciton is good for the generation of carrier by comparing the relaxation behavior of WS₂ excited to A and B exciton states. However, the bound exciton relaxation, originating from the capture process of the defect state, would exist and play an unfavorable role during the functioning of devices.