Abstract: Salinization represents a critical environmental challenge in arid regions, severely constraining agricultural productivity and compromising the carbon sequestration capacity of ecosystems. The subsurface pipe drainage (SPD) technology is widely regarded as an effective engineering measure for ameliorating saline-alkali soils. However, a comprehensive and systematic assessment of its net environmental impac-specifically, the trade-off between its life-cycle carbon costs and the resulting ecosystem carbon benefits-remains notably absent. This knowledge gap raises significant concerns regarding the technology's environmental sustainability and its role in climate change mitigation strategies. To address this, our study was conducted in the Yanqi Basin of Xinjiang, a representative arid inland region plagued by severe secondary salinization. We aimed to quantify the complete carbon balance of SPD systems and elucidate the underlying synergistic mechanisms linking water-salt regulation with the soil-crop-carbon sequestration system. A field experiment was established with four distinct treatments: a control employing conventional flood irrigation (CK) and three SPD treatments with varying design parameters (T1: burial depth 1.4 m, spacing 20 m; T2: depth 1.6 m, spacing 20 m; T3: depth 1.6 m, spacing 40 m). We monitored seasonal dynamics of soil salinity, measured sunflower (Helianthus annuus L.) yield at harvest, and quantified the vegetation carbon sink. Crucially, a holistic life-cycle carbon accounting framework was constructed and applied. This framework systematically integrated direct carbon costs, encompassing emissions from material production, on-site construction activities, and system operation, with the indirect carbon net changes occurring within the ecosystem. The results conclusively demonstrated the multi-faceted efficacy of SPD. All SPD treatments significantly reduced rootzone (0-60 cm) soil salinity by 53% to 67% relative to CK, effectively breaking the salt stress on crops. This improvement in the rhizosphere environment directly translated to a substantial boost in agricultural productivity. Sunflower yields under SPD treatments reached 1965 to 2941 kg/hm², representing a remarkable increase of 139% to 257% over the CK yield of 823 kg/hm². Consequently, the enhanced biomass production drove a major increase in the vegetation carbon sink, which rose to 13431 to 17539 kg CO₂e/hm², an 89% to 146% enhancement compared to CK. Statistical analysis confirmed a strong "desalination-yield increase-carbon sequestration" chain effect, with significant negative correlations between soil salinity and both yield and carbon sink. Among the tested designs, the T1 treatment (1.4 m depth, 20 m spacing) consistently achieved the optimal ecological-economic balance, delivering effective desalination with superior carbon efficiency. The carbon accounting revealed that while the SPD system incurred direct carbon costs ranging from 5310 to 5820 kg CO₂e/hm², these were overwhelmingly offset by the substantial vegetation carbon sink increment of 10113 to 10421 kg CO₂e/hm², resulting in a strongly positive net carbon balance. This study provides robust evidence that strategically designed SPD technology can synergistically enhance both the productive capacity and the ecological function, particularly the carbon sequestration potential, of saline-alkali lands. Our findings offer vital technical parameters and a solid scientific foundation for integrating scalable saline land remediation projects into regional and national carbon neutrality pathways.