The high thermal sensitivity of upconversion nanoparticles (UCNPs) luminescence at the nanoscale enables precise and rapid temperature detection, advancing nano-thermometry applications in quantum optics, catalytic reactions, and nanophotonics. Here, the phonon-assisted upconversion luminescence with a high thermal sensitivity is achieved by rare-earth doped UCNPs (NaYF4 : Yb3+/Er3+) in vacuum optical tweezers. We found that an essential contribution of phonon-assisted luminescence processes leading to high thermal sensitivity is not only from increasing the absorption cross section of Yb3+ but also from facilitating efficient energy transfer from Yb3+ to Er3+ ions. The increase in thermal sensitivity with the number of phonons involved in the progress of phonon-assisted luminescence can be well described by the temperature-dependent phonon population model. The study reveals how phonon-assisted processes influence thermal sensitivity in UCNPs, contributing to a deeper understanding of nanoscale temperature-dependent luminescence.