The ion-doped ceramic crystal is different from the single crystal in the respect of theoretical calculation method, because of the presence of grain boundaries. The populations at the active levels in both CW and pulsed regimes each as a function of the concentration of active impurities are deduced by means of the luminescence decay curve. Taking the Nd3+:YAG laser ceramic for example, we calculate the optimal doping concentration and compare our calculated results with reported experimental data, showing that our theoretical calculation is in line with the trend of the experimental data, and also we compare our results with the Nd3+:YAG single crystal theoretical data. showing that the ceramic crystal is superior to the laser doped laser crystal in the respect of population.