Potassium is an essential nutrient element for the growth of cotton，the effects of different soil potassium levels on the photosynthesis and yield of cotton were investigated through field experiments，so as to provide theoretical support for the potassium management in cotton field. The experiment was conducted at the experimental farm of the Cotton Research Institute，Chinese academy of Agricultural Sciences in Anyang county，Henan province. Split plot was designed，the main treatments included three soil potassium levels，i.e. high potassium(135.6 mg/kg available potassium)，medium potassium(123.37 mg/kg available potassium)，low potassium(109.73 mg/kg available potassium)，the sub-treatments were three different cotton varieties including early varieties(Zhongmian no.50)，early to medium varieties(Lumian No.28)，medium varieties(Xinmian 99B). The net photosynthetic rate，SPAD value，leaf area and potassium concentration of 10 cotton plants were determined at seedling，bud and flower and boll stages respectively，and the seed cotton yield was harvested and determined at maturity stage. The results showed that there was no significant difference in leaf area under different potassium levels in the seedling stage. But in flower and boll stage，the leaf area of cotton treated with high potassium was significantly higher than that treated with low potassium，and there was no significant difference from that treated with medium potassium. In seedling stage and bud stage，soil potassium level had no significant effect on chlorophyll SPAD value and net photosynthetic efficiency of functional leaves of the three cotton varieties，but there was significant difference in flowering and boll stage，which showed high potassium>medium potassium > low potassium. At seedling stage，bud stage and flower and boll stage，potassium contents in different organs of cotton of three cultivars were significantly different at different potassium levels，which was manifested as high potassium > medium potassium > low potassium. Under high soil potassium treatment，the seed cotton yields of the three cotton cultivars were significantly higher than that under medium potassium and low potassium treatment，but the yield difference between medium potassium and low potassium treatment was not significant. To sum up，insufficient soil potassium mainly affects the potassium content of each organ of cotton plant，leaf area，chlorophyll content and net photosynthetic rate，resulting in the decline of seed cotton yield. However，cotton varieties with a shorter growth period can better coordinate vegetative growth and reproductive growth to ensure the yield formation under the condition of low potassium.