Abstract: Potassium (K) is the most important cation in plants, which plays an important role in plant growth, metabolism and yield. K-deficient soils are common world-wide, and their widespread occurrence in the crop growing regions of northeastern China has been recognized in these years. Nowadays, the occurrence of K deficiency in soybean (Glycine max L.), the most important oil crop in northeastern China, has increased. This article presents the responses of photosynthetic characteristics and active oxygen metabolism of four K-tolerance soybean varieties, Shennong 6 and GD1617 as tolerant and Tiefeng 31 and Tie 95068-5 as sensitive, to low potassium stress under field condition from seedlings to maturity stage. The results showed that the leaf net photosynthetic rate (Pn), chlorophyll (Chl) content and stomatal conductance (Gs) of K-tolerant varieties decreased under low potassium stress, but compared with the sensitive varieties, the tolerant varieties decreased less, while intercellular CO2 concentration (Ci), transpiration rate (Tr) and content of praline content increased with sensitive verities increased more. The SOD activities of tolerant varieties were lower than those of sensitive one at flowering stage, while higher at maturity stage. The CAT activities of tolerant varieties were significant higher than those of sensitive varieties at both flowering and podding stages. The POD activities of tolerant varieties were significantly higher than those of sensitive varieties in maturity stage. In latter stage, the contents of MDA of tolerant varieties were lower than those of sensitive varieties.