The infiltration pressure field of fiber reinforced AZ91D was studied in this paper. The carbon fiber T300 was selected and covered with rough TiO2 coat by means of plasma techniques. These carbon fibers were combined together and a kind of continuous pre-former with many micro-passes was constructed. When this pre-former went though a infiltration cabin filled with molten magnesium alloys, the melt infiltrated into pre-former through those micro-passes under infiltration pressure. A physical model and related mathematic model were built to relate this process. The relationship among pressure (p), infiltration flow rate change (e) and the location of infiltration flow (r) was molded.