Polyimide (PI) composites cross-linked through octa(aminophenyl)silsesquioxane (OAPS) were thermally cured with a controlled temperature program. The compatibility between OAPS and PI was good. FTIR analysis indicated that the imidization reaction was complete. TGA results indicated that OAPS did not change the decomposition path of PI. The incorporation of OAPS into PI improved a little the tensile strength and Young's modulus. The fire resistance performances of the PI/OAPS composites were evaluated. The LOI tests showed that incorporation of OAPS into PI unproved the flame retardancy significantly. The mechanism of the flame retardancy of the composites was studied. The volatile pyrolysis products of PI/OAPS composites were similar to those of the PI control. However, the decomposition products of OAFS remained in the condensed phase and improved the thermo-stability of the char efficiently. The out-migration effect of silicon was beneficial to protect the compact char.