In this article, we report the local heating effects induced by laser irradiation in mechanically exfoliated WS2 monolayers and the characterization of the resulting inhomogeneous temperature field with the technique of photoluminescence spectroscopy. By comparatively measuring the PL spectra of the monolayer WS2 in different contact conditions as well as in air and vacuum, we confirm the existence of remarkable heating in the monolayer due to laser irradiation. In particular, it is found that a specific excitonic emission peak could be used to characterize the inhomogeneous Gaussian-like thermal field and the heat transfer behaviors. Based on the dependence of the excitonic emission energy on temperature, the local temperature induced by laser irradiation may be obtained, i.e., a local temperature up to 213 °C was found at the central region of the irradiated area in vacuum for the excitation power of 450 μW. Such optical method may be applied to the measurement of local heating effects in 2D semiconductors and relevant van der Waals devices.