Abstract: 【Objective】 To explore the effects of drought stress on photosynthetic characteristics of sugar beet root during expansion period. 【Methods】 Two irrigation treatments were set up with H1352 (drought-tolerant type) and 81GM26 (drought-sensitive type) sugar beet varieties, and the maximum net photosynthetic rate (Pnmax), dark breath rate (Rd), apparent quantum efficiency (AQY), light compensation point (LCP) and light saturation point (LSP) main photosynthetic parameters were obtained by simulating the response curves of leaf net photosynthetic rate and light intensity.【Results】 The results showed that the leaf Pn of H1352 and 81GM26 sugar beet varieties increased linearly with the increase of PAR, then increased rapidly, and finally became stable or decreased. Under different PAR conditions, the net photosynthetic rate of the leaves of the two varieties was higher than that of drought stress treatment, and with the increase of PAR, the difference between the two treatments reached a very significant level. Drought stress reduced the maximum net photosynthetic rate and light saturation point of the leaves (P<0.01). The difference among the treatments was very significant. The maximum net photosynthetic rates of H1352 and 81GM26 under drought stress were 24.8 and 21.0 μmol/(m²·s) , respectively, which were 18% and 26% lower than those of the control, respectively; The light compensation points were 600 and 601 μmol/(m²·s) , respectively, which were 18.5% and 16.4% lower than that of the control, respectively. The light adaptation range of the leaves was narrowed and the light energy utilization ability was reduced. Under the drought stress condition, the dark respiration rate of H1352 variety decreased significantly, which was 40.3% lower than that of the control, while there was no significant difference between the treatments of 81GM26 variety.【Conclusion】 When sugar beet root tuber was exposed to drought stress with water holding capacity 40% -45% field, the maximum net photosynthetic rate, dark respiration rate and light saturation point of sugar beet decreased, the light compensation point and apparent quantum efficiency varied with varieties, the range of light energy utilization narrowed and the ability of light energy utilization weakened. Under drought condition, the photosynthetic capacity of drought-tolerant variety H1352 was lower than that of drought-sensitive 81GM26.