Abstract: Guar Cyamposis Tetragonoloba(L.) Taub. is an important raw material of guar gum.Galactomannan,an active ingredient of the gum,is generally found to be negatively correlated with seed yield of guar.An experiment was conducted in pot to determine the effects of four potassium(K) fertilizer level K1=0 (control),K2= 8.952、K3=13.381 and K4=17.857 mg/kg soil on guar photosynthesis,chlorophyll content,soluble protein content of leaf,and endospermous carbohydrates accumulation.Supplying K significantly increased protein and chlorophyll content,enhanced net photosynthetic rate(NPR) of leaf,and extended high photosynthetic duration of guar leaf.Sucrose and glucose content in the leaf showed no difference under different K treatments in the early days.60 days after emergence,however,sucrose and glucose content in the leaf treated with K fertilizer were lower than that of the control.K increased total soluble sugar,rhamnose,galactose,sucrose and gross polysaccharide content in the seed significantly compared with the control.The highest grain yield was found in K3,while the lowest was found in control and K4.Galactomannan relative content in the seed endosperm was the highest for K2,followed by K3,control and K4.The gross sugar and GM produced per plant in K3 was the highest due to its highest yield obtained.All the results indicated that K fertilization could enhance leaf photosynthesis,accelerate the transportation of sugar from leaf to seed,promote carbohydrates accumulation,and increase grain yield.