The resources and environment carrying capacity is an important basis for measuring the resource conditions of a region and guiding the spatial distribution of land. In the current evaluation research of resources and environment carrying capacity, we are faced with problems such as the unclear dynamic mechanism of the carrying capacity and the insufficient dynamic evaluation of the resource elements. In order to objectively and accurately evaluate the resources and environment carrying capacity of small and medium-sized county units under the requirements of high-quality development, this article, from an open system perspective, reflects the spatial and temporal dynamic properties of carrying capacity and explores the coupling mechanism between the four subsystems of resources, environment, socio-economy, and population. Then it discusses the impact of resource flow on the carrying scale and carrying quality of resource inflow and outflow areas. Based on the exploration of the dynamic carrying mechanism of resources and environment carrying capacity, a theoretical framework for the dynamic evaluation of resources and environment carrying capacity at the county scale is constructed. Finally, the framework is applied to Wuqing district of Tianjin to test its applicability. Empirical research results show that, compared with static evaluation, the dynamic carrying capacity evaluation results are closer to the real situation of local resources, environment and socio-economic level, which, to a certain extent, can support the decision-making of territorial spatial planning at the local level and the optimization of resource space at the regional level.