In this study, we address the "mismatch between population and land" conflict, resulting from unbalanced population and land urbanization, which has become a key impediment to enhancing carbon sink functionality in China's terrestrial ecosystems. We establish a hybrid analytical framework combining Geographically Weighted Random Forest and Structural Equation Modeling to systematically analyze the spatial-temporal heterogeneity and complex causal driving mechanisms of China's terrestrial vegetation carbon sinks. Our findings reveal several key points: (1) The carbon sink of urban vegetation in China exhibits a distribution pattern of "high in the south and north, low in the middle". Regions such as Central China, North China, and the northern part of East China show a catching-up trend of "low base, high growth". Meanwhile, the eastern part of South China and the southern part of East China face sustainability challenges of "high background, negative growth". (2) Land urbanization is the core driver suppressing vegetation carbon sinks, with its negative impact significantly stronger than that of population urbanization, and its marginal effect continuously strengthening over time. (3) The impact pathways of urbanization are dual-natured. The direct negative impact of population urbanization is moderated by its indirect positive effects, with the total path coefficient weakening from -0.1175 to -0.0556. In contrast, the negative effect of land urbanization is amplified by its indirect pathways due to disorderly expansion, with the total path coefficient increasing from -0.1847 to -0.2983. (4) The driving mechanisms differ fundamentally across cities with different development models. In "population-land mismatched" cities, population urbanization shows a unique positive driving potential, with a total path coefficient of 0.0618, while land urbanization remains the core inhibitory factor. In "population-land coordinated" mature cities, both types of urbanization transition into strong negative suppressors, marking a new stage of urban development that must rely on intensive and refined green development. Our conclusions provide empirical support for building differentiated urban carbon-neutral pathways, suggesting that while strictly controlling disorderly land expansion, appropriately guiding population agglomeration in "population-land mismatched" cities can provide a scientific basis for synergistically advancing new-type urbanization and the national "dual carbon" strategy.