Semi-solid AlSi9Mg alloy melt and corresponding standard test specimens were prepared by rheo-diecasting process on incline cooling and shearing bench. Then the specimens were tested on their mechanical properties. The characteristic parameters of primary solid phase (including solid fraction, diameter of grains and shape factor) were analyzed through quantitative metallographic techniques.The influence of fulling velocity on the semi-solid microstructures and properties of AlSi9Mg were researched into. The results show that:the primary solid fraction and shape factor of AlSi9Mg alloy first increase and then decrease, and the average diameter of grains becomes smaller at first and then becomes bigger along with the increase of the filling velocity. Simultaneously their mechanical properties change significantly, the Brinell hardness increases along with the filling velocity, but the tensile strength and impact toughness increase respectively to 275MPa and 129.8kJ/m², and then begin to decline. In this experiment, a reasonable rate of filling is about 1.9m/s.