Population dynamics, an important problem in ecology and demography, is nothing but a positive feedback, so it is natural to study it by feedback control technique. In this paper the fundamental relation and characteristic equation for population systems are derived with reference to an improved Lexis diagram based on the concept of feedback. These results are then confirmed by differential equations on the analogy of a belt conveyor process. It is concluded that the population system is a dynamic system with input, output and positive feedback, and can be treated in the same way as an engineering system with distributed parameters by the technique developed in feedback control engineering.