Marx's significant discussions on problem of population production are scattered in a series of his works, especially in Capital and its manuscripts. Research reveals that through the preliminary construction of historical materialism in German Ideology, the critique of classical and vulgar economics' population theories in the Capital manuscripts, and further research and elaboration in Capital itself, there exists a theory of population production embedded in Marx's critical system of political economy. Marx's theory of population production can be summarized as follows: one core law, three pairs of important relationships, and three key issues. Trying to explore and construct Marx's theory of population production is not only the need of innovating and developing Marxist political economy, but also the need of explaining and responding to practical problems.