Phonon-assisted up conversion photoluminescence (UPL) is an anti-Stokes process emitting photons of energy higher than the excitation photons, with up conversion energy gain provided by optical phonons. Atomically thin transition metal dichalcogenides provide a promising platform for exploring the phonon-assisted UPL process due to their strong phonon-exciton interactions. Here, high-temperature phonon-assisted UPL process in monolayer WSe2 is investigated, aiming to understand the role of phonon population and the number of phonons involved in the UPL process at elevated temperatures. It is demonstrated that the integrated intensity of UPL emission significantly increases by two orders of magnitude as the temperature rises from room temperature of 295 to 476 K, which is distinguished from the photoluminescence emission usually suffering from thermal quenching. The observed growth of UPL emission intensity is attributed to both the increased phonon population and the reduced number of phonons required at elevated temperatures. Our study paves the way toward near-infrared light detection, anti-Stokes energy harvesting, optical refrigeration, and temperature sensing.