Abstract: Against the backdrop of China’s national low-carbon development strategy, developing sustainable irrigation districts has become a critical objective; however, current agricultural carbon accounting frameworks often inadequately address emissions from infrastructure construction and renovation. To bridge this gap, this study conducted a quantitative lifecycle assessment (LCA) of three prevalent irrigation water conveyance systems, namely Concrete block masonry channel (CBMC), U-shaped precast concrete channel (UPCC), and Low-pressure polyethylene pipe (LPP), in a typical small-scale irrigation district within China’s southern water network region. The assessment adopted an LCA methodology integrated with the emission-factor approach, defining a system boundary that encompassed four consecutive stages: material production, transportation, construction, and operation. Activity data, including detailed material quantities, transport distances, machinery usage, and energy consumption, were derived from engineering design specifications, while carbon emission factors for key materials (concrete, steel, timber, polyethylene) and energy sources (diesel, grid electricity) were sourced from authoritative Chinese national standards and peer-reviewed literature. The functional unit was the total carbon dioxide equivalent emissions over the projected service life, with results also presented per unit irrigated area per year to aid comparison and scaling. Results indicate that, across the four lifecycle stages—production, transportation, construction, and operation—the production phase contributes 26.12% to 57.41% of total emissions; transportation accounts for the smallest share (0.11% ～ 1.57%); construction contributes 0.46% ～ 4.70%; and operation dominates with 38.37% ～ 73.31%. The findings identified material production and operational energy use as the dominant emission sources. Specifically, concrete was the primary carbon source for CBMC and UPCC, accounting for 53.08% and 17.85% of their total lifecycle emissions, respectively, whereas polyethylene contributed 21.80% for LPP. During operation, electricity for pumping constituted the most significant factor, representing 38.24%, 68.03%, and 73.28% of total emissions for CBMC, UPCC, and LPP, respectively. In aggregate, the LPP system demonstrated a markedly lower carbon footprint of 399 392.85 kg, compared to 1 000 875.28 kg for CBMC and 537 746.62 kg for UPCC, underscoring its substantial emission reduction potential alongside recognized advantages in water and land savings. Furthermore, UPCC required only about one-fifth of the concrete volume used in CBMC, leading to a 66.97% reduction in emissions between the two channel types, highlighting the benefits of precast technology. Sensitivity analysis revealed that pump service life was the most influential parameter on the total carbon footprint, followed by material transport distance. With an increase in the lifespan of irrigation pumps, the annual carbon emissions allocated from the production, transportation, and construction phases of the irrigation water delivery system are reduced, leading to a corresponding decrease in the annual carbon emission intensity per unit area. These results emphasize that strategic material selection, structural design optimization, and adoption of prefabricated elements are crucial for reducing embodied carbon, while a transition to renewable energy for pumping could cut operational emissions by over 90%. These insights, derived from a focus on core conveyance components, suggest that future assessments could expand the system boundary to include ancillary structures and end-of-life phases for enhanced comprehensiveness. In conclusion, this research establishes a practical LCA-based model, confirming the significant decarbonization potential of low-pressure pipe irrigation and prefabricated concrete channels, thereby providing an evidence-based foundation for selecting low-carbon materials, optimizing engineering designs, and integrating clean energy in irrigation district renovation projects to advance sustainable agricultural water management.