Bismaleimide resin (BMI) has good processing properties and excellent heat resistance. Composites based on BMI have been widely used in aerospace field in recent years. In this study, T800/BMI composites were cured by autoclave process. Laminates with different void content of [0/90/±45]_2s and [0]₁₄ layers were prepared by setting the forming pressure of 0.1~ 0.8 MPa. The void content, size, shape and distribution were characterized by microscopy. The tensile, bending and interlayer shear tests were carried out for the mechanical properties, and the failure morphology of the interlayer shear samples was observed by scanning electron microscopy. The results show that low forming pressure will cause void mutation, void size increase, and distribution from intralayer to interlayer, resulting in a sharp decrease in mechanical properties, among which the interlayer shear performance is most affected by void mutation. When the molding pressure is reduced from 0.8 MPa to 0.1 MPa, the interlayer shear strength of the [0/90/±45]_2s specimen is decreased by 26.22%, which is due to the large-size voids between the layers.