Abstract: 【Objective】 In order to clarify the variation of photosynthetic characteristics of sweet sorghum at different growth stages. 【Methods】 The optimal topdressing method of nitrogen fertilizer under different topdressing treatments of nitrogen fertilizer, four different topdressing treatments, such as control without fertilization(CK), B₁(2 times furrow top dressing), B₂(2 times dropwise top dressing), B₃(4 times dropwise top dressing), were set and the effects of treatments on the photosynthetic characteristics, water use efficiency(WUE)and biomass yield of six sweet sorghum cultivars at different growth stages were studied. 【Results】 The experimental results showed that the changing law of Pn, Tr, Ci and WUE in sweet sorghum leaves under different topdressing treatments at different growth stages varied between varieties and treatments.The photosynthetic characteristics of the same variety at the same growth stage under different treatments were different, and the photosynthetic characteristics of leaves were affected by topdressing of nitrogen fertilizer.The change range of Pn was at 5.50-28.52 μmol CO₂/(m²·s), which was the highest value of Dalishi in B₃ treatment, which was significantly higher than CK by 39.91%.The difference between the Tr value of all varieties in B₂ and CK was not significant.At jointing and milk ripening stage, there was no significant difference in the Gs values of all cultivars between treatments and CK.The variation range of WUE was at 3.05-16.86 μmol/mmol.Compared with CK, only the WUE value of Jintianzai No.3 in B₂ treatment was significantly increased(71.28%)while those of other treatments were not significantly different from CK at the jointing stage.At the flowering stage, the WUE values of all varieties in B₁ were not significantly different from that in CK.The biomass yield ranged from 47.51 t/hm² to 85.85 t/hm², and the yield increase of treatments was 7.66%-53.67%, compared with CK.The yield of Dalishi was higher than that of other varieties in all treatments, and the order of the highest biomass yields in B₁, B₂ and B₃ was the same, and that was Dalishi > Jintianzai 3 > Liaotian 3. 【Conclusion】 Dalishi, Jintianzai 3 and Liaotianzai 3 had good adaptability in this region, and B₂ treatment can improve the photosynthetic conditions and maximize the yield and water use efficiency. This treatment was the best nitrogen topdressing mode to promote the growth and development of sorghum in arid areas.