In order to explore the changes of soluble sugar content in root bleeding sap during the genetic improvement of soybean varieties and its relationship with the plant photosynthesis, 22 soybean varieties bred in Jilin Province during 1923-2004 were measured to study the change of soluble sugar content in root bleeding sap and its relationship with the net photosynthetic rate (Pn) of the function leaf in 2010 and 2011. The results showed that while root bleeding sap mass increased with genetic improvement of soybean cultivars, similar to the soluble sugar content in roots bleeding sap, and the correlation of soluble sugar content with year of release reached a significant positive level at flowering stage (R2). Maximum soluble sugar content was measured at R2, while at pod growth stage (R4) and seed filling stage (R6), soluble sugar content in root bleeding sap were extremely low. The relevant analysis of soluble sugar content of bleeding sap and Pn at R2, R4 and R6 showed that soluble sugar content was significantly correlated with Pn at R2, but the correlation was not remarkable at R4 and R6. It was suggested that R2 soluble sugar content in root bleeding sap could be used as an index of plant photosynthetic capacity.