To enhance the comprehension on the internal rule of microstructure evolvement and quality-controlled relativity, the microstructure during the induction reheating of billets and die casting of parts was systematically investigated. According to general structure observation and aided analysis of some computer simulations, the quantitative relationships between microscopic morphology (including solid fraction and grain shape) and formation state was minutely discussed. The experimental results showed that liquid and solid phase in semi-solid slurry had different filling tendency, fluid velocity of liquid phase in the area of high filling speed was relative quick and easy to flow into far area and corner. Solid particles far from gate possessed fine and round grain. Moreover, the structure configuration of original billets markedly affected forming process; billets with fine and round grain were required for the die-castings of complex shape and thin thickness. In the case of the big change in shape and thickness of die-castings, the design of gating system and the control of plunger speed should be so properly carried out as to avoid unequal distribution of solid and liquid phase in die-casting part as soon as possible.