In order to clarify the influence of processing parameters on the characteristics of buckwheat extrudates， to understand the relationship among different characteristics of buckwheat extrudates， and to explain the characteristics from the molecular structure level， buckwheat flour was extruded under different water content， processing temperature and screw speed by DSE-25 Twin-screw Extruder using response surface design. The physicochemical properties of buckwheat extrudates such as expansion index， water absorption index， water solubility index， and visocsity were studied systematically. The results showed that processing temperature and screw speed were important factors influencing the sectional expansion index， water solubility index and water absorption index. Moisture content and screw speed were important for the viscosity， the average molecular radius and the average molecular weight. There was a significant negative correlation between the specific mechanical energy and the starch average molecular size. Starch average molecular size was negatively correlated with water solubility index and total color difference， and positively correlated with water absorption index and viscosity. Water solubility index could be used to estimate water absorption index， and average molecular radius of the starch can be used to estimate weight average molecular weight. Starch molecules were more degraded when shear is more severe， with reduced average molecular size. With higher water content， lower barrel temperature， and smaller screw speed， special mechanical energy would be smaller， starch average molecular size would be larger， leading to higher water absorption index， viscosity， and lower water solubility index and color difference. Extrusion can be used to alter the starch structure， and the physicochemical properties such as water solubility， water absorption， viscosity can be changed accordingly.