Phonon-assisted upconverted emission lies at the heart of energy harvesting, bioimaging, optical cryptography and optical refrigeration. It has been demonstrated that the emerging two-dimensional (2D) semiconductors can provide a great platform for efficient phonon-assisted upconversion due to the enhanced optical transition strength and phonon-exciton interaction of 2D excitons. However, the research on the further enhancement of excitonic upconverted emission in 2D semiconductors is almost blank. Here we report the enhanced multiphoton upconverted emission of 2D excitons in doubly resonant plasmonic nanocavity. Owing to the enhanced light collection, enhanced excitation rate and quantum efficiency enhancement arising from Purcell effect, the upconverted emission amplification of > 1000 folds and the decrease of 2 ~ 3 orders of magnitude for saturated excitation energy density are achieved. These findings pave the way to the development of excitonic upconversion lasing, nanoscopic thermometry and sensing, and open up the possibility of optical refrigeration in future 2D electronic or excitonic devices.