A series of isocyanate-based flexible polyimide foams were prepared with different isocyanate indices. The structure and.properties of these polyimide foams were characterized in detail. The results indicated that with the increase of isocyanate index, the density decreased from 8.7 kg/m(3) to 5.7 kg/m(3). The open cell ratio was in the range of 94.9%-96.4% and less influenced by the content of isocyanate. The compression strength increased first with the increase of isocyanate index, and reached the maximum value at an isocyanate index of 1.04. All the polyimide foams possessed excellent thermal stability, with temperature at weight loss of 5% and 10% being in the range of 288.5-320.0 degrees C and 337.7-369.0 degrees C, respectively. The limiting oxygen index (LOI) values for all the samples were above 25%, indicating the high flame resistance character of polyimide. The heat release peak values, the total heat release and mean heat release rate exhibited similar trends of little changes with isocyanate index varying from 0.89 to 1.04 and increasing rapidly with the index more than 1.11, indicating the side products of amino or uramido produced by excess isocyanate would damage the flame resistance properties of polyimide foams severely.