Traditional foundation pit support systems typically entail high carbon emissions and elevated costs. To address these challenges, this study proposes a novel green support system utilizing agricultural waste in the form of solidified cotton straw grid piles. The mechanical and environmental performance of the system was systematically evaluated through laboratory bending tests, field tests, and assessments of cost and carbon emissions. The results indicated that the cotton straw grid increased the flexural capacity by approximately 20% compared with straw bundles of equivalent mass, significantly enhancing flexural performance. Field tests showed that the maximum horizontal displacement and settlement remained below 4.2 mm, well within allowable limits specified in relevant design standards, confirming structural stability. Relative to conventional bamboo- and steel-reinforced cement-soil piles, the proposed system reduced construction costs and carbon emissions by over 9.9% and 47.0%, respectively, demonstrating clear economic and carbon reduction benefits. This study provides a feasible pathway for the utilization of cotton straw and offers a sustainable and cost-effective alternative for foundation pit support, contributing to low-carbon development in geotechnical engineering.