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Key to regional sustainable development are the development and interplay of population dynamics and social welfare, each playing a significant role. As a representative region with demographic characteristics such as negative population growth and large labor outflow, the development and interaction between population and social welfare in Nanchong deserve in-depth exploration. This article takes the development of population and social welfare in Nanchong as the research object, and constructs an evaluation indicator system of population and social welfare through research backtracking, and uses entropy method and coupling coordination model to measure the development level and interactive effect of population and social welfare in Nanchong from 2010 to 2021. The research results show that: Firstly, the comprehensive evaluation results of population in Nanchong shows a linear upward trend, which indicates the stable positive effect of population structure and distribution, the gradual improvement effect of population quality effectively compensate for the weakening effect of population quantity, thus achieving the positive development of population. Secondly, the comprehensive evaluation results of social welfare in Nanchong shows an exponential upward trend, which indicates the social welfare has maintained a rapid growth momentum in various dimensions and the long-term positive effects have completely absorbed the negative effects, thus achieving the positive development of social welfare. Thirdly, during the sample period, the population and social welfare in Nanchong consistently maintained a high level of interaction strength, with factors diffusing and integrating. On this basis, the diffusion theory is used as an empirical reference to construct three interactive mechanisms between the population and social welfare in Nanchong and the implications are inferred from the empirical results.人口动态与社会福利的发展及其相互作用，是区域可持续发展的核心要义，二者均发挥着重要支撑作用。作为兼具人口负增长、劳动力大规模外流等人口特征的典型区域，南充市人口与社会福利的发展及互动关系值得开展深入探究。本文以南充市人口与社会福利发展为研究对象，通过研究回溯构建人口与社会福利评价指标体系，并采用熵权法（entropy method）与耦合协调模型（coupling coordination model），测度2010-2021年南充市人口与社会福利的发展水平及互动效应。研究结果表明：其一，南充市人口综合评价结果呈线性上升趋势，这表明人口结构与分布的稳定正向效应、人口质量的逐步改善效应，有效抵消了人口数量的弱化影响，进而实现人口的良性发展。其二，南充市社会福利综合评价结果呈指数上升趋势，表明社会福利在各维度均保持快速增长势头，长期正向效应已完全抵消负向效应，进而实现社会福利的良性发展。其三，样本考察期内，南充市人口与社会福利始终保持高强度互动水平，要素实现扩散与融合。在此基础上，本文以扩散理论为实证参照，构建南充市人口与社会福利的三类互动机制，并基于实证结果推演相关启示。