Faced with the increasingly prominent contradiction between man and water and based on the theory of water footprints, this paper constructs a watershed lexicographic optimal allocation model pertaining to population quantity and proposes an Index of Water Supply and Demand（IWSD）model . The objective is to comprehensively identify the functional mechanisms of population structure in the lexicographic optimal allocation of water footprints. The results show: the population quantity has a significant positive impact on the lexicographic optimal allocation of water footprints. The intensity of human activity is a key factor affecting the regional IWSD. The lexicographic optimal allocation scheme of watershed water footprints based on population quantity has significant urbanrural dual structure characteristics, which essentially reflects the spatial difference state of urban and rural infrastructure and industrial structure. At the same time, the optimal allocation scheme also has the characteristics of employment structure, which essentially reflects the spatial difference state of industrial water use efficiency.