Over the past decades considerable efforts have been put forth to fabricate lightweight polyimide (PI) foams, considering their superior heat-resistance, flame retardancy, less smoke generation, which make them widely used in many advanced applications, such as aerospace, submarine, special ships and high-speed trains. However, most PI foams have cell size in the range of millimeters. The mechanical properties of these foams are usually much weaker than those of the solid polymers. Microcellular structure endows polymeric foams with the improved mechanical properties. However, the preparation of lightweight thermoplastic PI microcellular foam with a large size is challenging and inefficient, because of low gas solubility, high polymer matrix, and an extremely long saturation time. In this study, the mixed CO2/organic solvent was used as the physical blowing agent, the modified PI was selected as the resin, and a solid foaming technique was applied to prepare PI microcellular bead foams with cell size of 60-100 m and super-low density of 20-40 kg/m3. Meanwhile, the prepared PI bead foam was expandable because a trace of blowing agent was still present after being aged a long period of time. With using a novel steam-chest molding process, the prepared expandable PI bead foams were molded into the 3-D shaped PI foam products (EPI) with the density of about 25 kg/m3. It was found that EPI presented low thermal insulation of 0.030 W/m.k, high compression strength of 0.21 MPa, high service temperature of 200 C, flame retardancy, and low smoke generation. The well-defined properties and the simple processing procedure ensure EPI sample potentionally use in many advanced thermal insulation fields.