The vacuum gas pressure infiltration was performed by 3D five-directional and 3D orthogonal braided performs woven by M40 graphite fiber composite (3D-Cf/Al) with the volume fraction of 51% and the density higher than 97%. After analyzing the morphology and formation mechanism of micro defects and control measures, and comparing the effects of braiding structures on micro defect formation, the results showed that the defects in the composite were micron grade micro defects, of which the main defects included pores within the fiber beam and partial fiber reunited, cold shut, microscopy shrinkage cavity and small inclusion in the substrate concentration. Of note the pores between the fiber beam and pores within the fiber beam of 3D orthogonal composite were less than 3D five-directional composite, while the fiber reunited phenomenon of 3D orthogonal composite was more serious than 3D five-directional composite. The main reason for the different defects was braided structure distinctions. By increasing the fiber preheat temperature could effectively reduce the internal pores defect and partial fiber reunited phenomenon.