Aims <p>Biochar has shown beneficial effects in agricultural production, but further exploration is needed on how the coupling of biochar and irrigation affects the formation of sugar beet source sink relationships in saline soil.</p> Methods <p>Through a three-year field experiment, the effects of three irrigation levels (I1:80%ETc (crop evapotranspiration), I2:100% ETc, and I3:120% ETc) and four biochar application rates (B0: 0 t ha⁻¹, B10:10 t ha⁻¹, B20: 20 t ha⁻¹, and B30: 30 t ha⁻¹) on sugar beet growth, nitrogen absorption, source-sink relationships, and yield were studied.</p> Results <p>The results indicated that full irrigation (I2) and biochar (B10 followed by B20) significantly improved stem thickness, root length, leaf area index (<i>LAI</i>), and photosynthetic parameters. Sugar beet yields increased by 19.6–28.3% under I2 and 9.7–18.2% under I3 compared to I1, and by 38.4–42.4%, 2.1–10.9%, and 17.2–28.5% under B10 compared to B0, B20, and B30, respectively. The B30 treatment increased sugar content by 3.6–15.2%, 3.9–16.1%, and 4.7–15.4% compared to other biochar treatments in 2021, 2022 and 2023, respectively. Source-sink analysis revealed that the combination of moderate irrigation (I2) and biochar amount (B10) optimized the coordination of source-sink in sugar beet, promoting dry matter accumulation and yield formation. Structural equation modeling identified leaf area index, tuber nitrogen accumulation, and maximum tuber growth rate as the main drivers affecting sugar beet yield. Relative chlorophyll contents were strongly correlated with photosynthesis and yield, highlighting the importance of managing water and nutrients.</p> Conclusions <p>Overall, full irrigation (100%ETc) combined with 10 t ha⁻¹ biochar application was effective strategy for enhancing sugar beet yield and quality.</p>