The puzzle of how internal OH- impurities affect photon energy upconversion has been quantitatively disentangled through combining internal OH- content manipulation in nanocrystals, spectroscopy and Monte Carlo simulation. The exponential relationship between upconversion luminescence intensity and the quantity of internal OH- was discovered. The work not only paves the way to pursuit new structures and/or doping patterns for higher upconversion efficiency, but also provides a new method for studying the internal defects of phosphors.