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Concentration quenching of rare-earth doped upconversion nanoparticles severely limits the dopant concentration, and this greatly hinders their potential applications. Therefore, it is necessary to understand the roles of dopant concentration in photon population and luminescence quenching for materials designed with improved upconversion luminescence (UCL). Herein, the excited-state dynamics of well-accepted NaYF4:Yb3+,Er3+ nanocrystals were investigated as models based on the Kohlrausch-function. The use of the Kohlrausch-function successfully disentangled the rise and decay of dynamics data and well revealed the kinetics. Photon population and concentration quenching mechanisms depending on the sensitizer concentration are deeply revealed by the regular variations of the fitting parameters. The results indicated that high doping of sensitizers will accelerate the population of both green and red emitting energy levels, but cause significant concentration quenching in green emission and little quenching in red emission. Our work opened up new pathways of kinetics analysis, which is beneficial for further mechanism development, and established detailed photon population and concentration quenching models depending on the doping concentration of the sensitizer.