Evaluation of pollutants effect at population level is crucial for realistic ecological risk assessment. Population growth rate has been used for many years as an index of population fitness. In recent, however, there is a growing concern about the importance of age structure or age distribution of population, together with the population growth rate in ecotoxicological researches. To estimate overall effect of heavy metals against on soil dwelling springtail species, Paronychiurus kimi, a predictive model was developed. The model combined a Leslie population projection matrix and the effect of heavy metals on reproduction rate of P. kimi. The age distribution of the springtail was separated into seven groups based on headcapsule widths and life history characteristics: one egg stage, three juvenile stages, and three adult stages. Components of the projection matrix were survival rate of each age stage, time to reproduction, and reproduction of adult stages. We constructed seven-stage population matrix model and predicted the age stage distribution compared to observed age distribution exposed to heavy metals. Heavy metals affected the population growth rate and the age distribution differently as the concentrations were changed. As the concentration increased, the proportion of old juveniles in the population increased gradually, whereas the fecundity decreased. Results from projection matrix simulation showed that the long term effects of toxicants were more severe than the predicted effects if the effect of age distributions were combined. Considering the age distribution is very critical to improve our understanding for realistic ecological risk assessment.