Abstract: The relationship between continuous and discrete models describing population evolution processes has been established. To accommodate the population policy being held in China we introduce a set of new definitions of quantities and functions and take the specific fertility rate to be the control parameter with psychological and stability constraints. An optimization problem to achieve desired population state is formulated, and a necessary condition for optimality has been proved by means of maximum principle, thus a computational method is provided for numerical study of planned population evolution processes that is needed in working out long term population policy.