Using a dynamic economic model with a life-cycle hypothesis, this paper examines the effect of population change on economy. Considering life expectancy, expected income, unexpected death, heritage and social endowment insurance, this model combines consumption life-cycle pattern with populationmodel and Solow growth model. The main idea of this model comes from Diamond's overlapping generations model. From the model study we found that with the decline of population mortality rate by age, saving rate increases. This finding modifies the main stream point that is saving rate declines due to population aging.