MoS2 has been studied intensively during recent years as a semiconducting material in several fields, including optoelectronics, for applications such as solar cells and phototransistors. The photoresponse mechanisms of MoS2 have been discussed but are not fully understood, especially the phenomenon in which the photocurrent slowly increases. Here, we report on a study of the photoresponse flash-light-processed MoS2 films of different thicknesses and areas. The photoresponse of such films under different light intensities and bias voltages was measured, showing significant current changes with a quick response followed by a slow one upon exposure to pulsed light. Our in-depth study suggested that the slow response was due to the photothermal effect that heats the MoS2; this hypothesis was supported by the resistivity change at different temperatures. The results obtained from MoS2 films with various thicknesses indicated that the minority-carrier diffusion length was 1.36 µm. This study explained the mechanism of the slow response of the MoS2 film and determined the effective thickness of MoS2 for a photoresponse to occur. The method used here for fabricating MoS2 films could be used for fabricating optoelectronic devices due to its simplicity.