Abstract: Drawing on micro-panel data covering 328 county-level administrative units and 164000 smallholder households across Guangxi, Yunnan, and Guizhou during the period 2021–2025, this study employs a two-way fixed effects model to empirically examine the impact of agricultural technology support on per capita net income growth among smallholders. The findings reveal four principal results. First, technological assistance exerts a statistically significant positive effect on household income growth; specifically, technical personnel support, project-based interventions, and infrastructure investment all contribute significantly, with infrastructure investment demonstrating the largest marginal effect. Second, terrain heterogeneity plays a pronounced role: the effectiveness of technology support varies markedly by geomorphic type, exhibiting an efficacy ranking of basin > hilly terrain > mountainous region. This underscores the theoretical imperative that technology extension strategies must align with regional resource endowments. Third, human capital and locational attributes serve as critical moderating factors. Labor skills and years of schooling enhance technological absorption capacity, whereas remote locations impede spillover benefits due to elevated market access costs. Fourth, compatibility between technology types and topographic conditions influences prioritization in dissemination. Through the “technology–topography–income elasticity” pathway, differentiated strategies are identified: mountainous areas should prioritize integrated pest management and compact water-saving equipment; hilly zones merit emphasis on water-efficient irrigation systems and slope-adapted machinery; basins are best suited for intelligent agricultural machinery and precision fertilization technologies. In conclusion, agricultural technology support constitutes an effective avenue for sustaining income gains among smallholders in Guangxi, Yunnan, and Guizhou. Nevertheless, maximizing its impact necessitates a terrain-specific approach that synergizes human capital enhancement with targeted mitigation of locational disadvantages. Precision interventions—such as deploying science and technology commissioners in mountainous counties or allocating high-tech projects to basins—should be implemented to optimize the allocation of technological resources and advance rural revitalization.