The solidification pressure is one of the most important factors in the counter-gravity casting process. Through testing and analyzing the microstructures of aluminum alloy castings under different solidification pressure, the effect of different solidification pressure on the secondary dendrite arm spacing and grain size is studied. The results show that with the increase of the solidification pressure, the secondary dendrite arm spacing and the grain size of aluminum alloy decreases. When the solidification pressure is 250 KPa, the secondary dendrite arm of aluminum alloy is thick, the SDAS is 37.9μm, but when the solidification pressure increases to 450 KPa, the refinement of grain is obviously, and the SDAS is 20.7μm, which is reduced by 45.3% comparing to solidification under 250 KPa. Moreover, when solidification pressure higher, the effect of feeding force becomes more evident, and the dendrite is broken when the feeding force higher than the strength of dendrite. Therefore, the grain size becomes more and more uniform and thin, and the (SDAS) of aluminum alloy are more and more small.