This paper studied the optimal biosynthesis conditions for water-soluble( 1 →3)( 1 →6)-α-D-glucan produced by glucansucrase using sucrose as substrate and its structure was investigated by modern analysis instruments. The optimal reaction conditions were as follows: temperature of 40 ℃,substrate concentration of 160 mg / m L,enzyme concentration of 10 U / m L,and reaction time of 1 h,and p H 5. 0. The HPSEC chromatograms of the polysaccharide appeared as a single symmetrical narrow peak. The weight-average molecular weight of this glucan was 2. 407× 107 g / mol,and the value of Mw / Mn was 1. 07. Results from FT-IR and1H-NMR suggested that this glucan was mainly composed of α-1,3 and α-1,6 linked D-glucopyranose units with ratio of approximately 3∶ 4.