Soil structure degradation from plastic film residue and rising costs necessitate green, film-free cultivation technologies in arid agriculture. This study investigated the regulatory effects of intercropped cumin density on soil coverage, photosynthetic carbon fixation, and yields in a cotton-cumin system without plastic film. Field experiments (2023-2024) compared single-cotton (CK) with three intercropped cumin seeding densities: 2.5 (D1), 3.85 (D2), and 5.2 (D3) kg & sdot;hm-2. During the symbiotic period, monocropping (CK) significantly increased cotton plant height (7.4-24.8%), stomatal conductance (Gs, 18.8-31.2%), and transpiration rate (Tr, 14.3-32.5%) over intercropped treatments. However, within intercropping, lower cumin density (D1) resulted in 18.5-29.2% higher total cotton plant carbon compared to D2 and D3. Conversely, cotton's net photosynthetic rate (Pn) in D2/D3 and cumin yield in D3 (28.6-43.1% above D1) were significantly greater. During the non-symbiotic period, intercropped cotton exhibited 4.4-10.7% greater stem thickness, 4.9-12.9% higher coverage, and 1.9-16.7% higher intercellular CO2 concentration (Ci) than CK. Among intercropping, D2 optimized cotton performance, significantly increasing coverage, Ci, and seed cotton yield by 4.9-5.1%, 4.6-16.7%, and 3.1-7.3%, respectively, compared to D1 and D3. Correlation analysis confirmed positive links between gas exchange parameters (Ci, Tr, Gs), cotton carbon content, and cumin yield. Under nonfilm deep-drip irrigation, a moderate cumin density of 3.85 kg hm-2 (D2) optimally balances interspecific competition and facilitation. By enhancing early growth coverage and bloom-stage photosynthetic capacity, D2 increased total cotton plant carbon by 9.5-15.2%, while boosting seed cotton and cumin yields by 4.1-7.3% and 33.5-43.1%, respectively. This density represents an optimal strategy for integrating productivity with carbon sequestration in sustainable, film-free cotton production in arid regions.