Soil water deficit is an important factor affecting the source–sink balance of sweet potato during its late-season growth, but water regulation during this period has not been well studied. Therefore, the aim of this study was to determine the appropriate irrigation level in late-season sweet potato, and the effect of irrigation level on accumulation and allocation of photosynthetic products. In this study, two yield-based field trials (2021–2022) were conducted in which five late-season irrigation levels set according to the crop evapotranspiration rate were tested (T0: non-irrigation, T1: 33% ETc, T2: 75% ETc, T3: 100% ETc, T4: 125% ETc). The effects of the different irrigation levels on photosynthetic physiological indexes,13C transfer allocation, water use efficiency (WUE), water productivity (WP), and the yield and economic benefit of sweet potato were studied. The results showed that late-season irrigation significantly increased the total chlorophyll content and net photosynthetic rate of functional leaves, in addition to promoting the accumulation of above-ground-source organic biomass (p< 0.05). The rate of13C allocation, maximum accumulation rate (Vmax), and average accumulation rate (Vmean) of dry matter in storage root were significantly higher under T2irrigation than under the other treatments (p< 0.05). This suggests that both non-irrigation (T0) and over-irrigation (T4) were not conducive to the transfer and allocation of photosynthetic products to storage roots in late-season sweet potato. However, moderate irrigation (T2) effectively promoted the source–sink balance, enhanced the source photosynthetic rate and stimulated the sink activity, such that more photosynthate was allocated to the storage sink. The results also showed that T2irrigation treatments significantly increased yield, WUE and WP compared to T0and T4(p< 0.05), suggesting that moderate irrigation (T2) can significantly promote the potential of storage root production and field productivity. There was a close relationship between economic benefit and marketable sweet potato yield, and both were highest under T2(p< 0.05), increasing by 36.1% and 59.9% compared with T0over the two-year study period. In conclusion, irrigation of late-season sweet potato with 75% evapotranspiration (T2) can improve both the yield and production potential. Together, these results support the use of late-season water management in the production of sweet potato.