Photosynthetic characteristics of sugar beet cultivars KWS0143 and Beta464 were studied under alkaline conditions, including 0 (A0, control), 25 (A1), 50 (A2), 75 (A3), and 100 mM of mixed alkali (Na2CO3:NaHCO3, 1:2). A2, A3, and A4 reduced net photosynthetic rate (PN), stomatal conductance, and transpiration rate, but rose intercellular CO2 concentration. Reduction in PN occurred probably due to nonstomatal limitation. The decrease in efficiency of photosynthetic electron transport might be the main reason for the decrease of PN. The concentrations of photosynthetic pigments were significantly reduced by high (A3 and A4) alkalinity. Changes in chloroplast ultrastructure are the reason for the decrease in chlorophyll content. Sugar beet could resist injury from alkali owing to osmotic substances and antioxidant enzymes if alkaline stress was at a lower level. The better performance of KWS0143 under alkalinity might be associated with its more efficient osmotic and antioxidant systems to resist injury of photosynthetic apparatus caused by alkalinity.