There exist amounts of surface blowhole,shrinkage porosities （holes） in produced aluminum alloy brake oil cylinder,resulting in seriously deteriorating the quality and production rate.Origination of defects occurrence in the brake oil cylinder was analyzed by the ProCast software.The gas hole in the castings is mainly attributed to lower pouring temperature and faster increasing velocity of filling surface.So for the brake oil cylinder with smaller cross section and flange structure,horizontal pouring can smoothly fill the cavity,effectively eliminating the defects resulting from the difficulty to exhaustion.Movable core can narrow the solidification time difference between the center and surface of the castings by greatly improving the heat emission conditions,obviously avoiding the occurrence of shrinkage porosities （holes）.