Polymethylacrylimide (PMI) foam is a kind of rigid foam with excellent thermodynamic properties, widely used in aerospace, sports equipment, and medical equipment. The foam properties of PMI foam are affected by the blowing agent. The type and amount of gas produced by the decomposition of blowing agent during heating have an effect on the size, morphology, and properties of PMI foam. The thermodynamic properties of self-foaming PMI foam using copolymerizable foaming monomers are directly affected by the foaming monomers. Therefore, in this work, acrylonitrile (AN) and methacrylic acid (MAA) were used as polymerization monomers, acrylamide (AM) as crosslinking agent, azodiisobutyronitrile (AIBN) as initiator, and tert-butyl methacrylate (tBMA) as copolymerizable foaming monomers. Lightweight and high strength PMI foam was prepared by free radical polymerization and free thermal foaming. The tert-butyl ester group in tBMA would thermally decompose into carboxyl groups and isobutene during the foaming stage, and the release rate of isobutene determined the physical structure of PMI and then affected its performance. Therefore, the effects of different foaming temperatures on the microstructure and thermodynamic properties of PMI foam were investigated. The physicochemical structure and properties of PMI foam were analyzed by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (SEM), thermogravimetric analyzer (TGA), differential scanning calorimeter (DSC), and universal testing machine. The results showed that with the increase of foaming temperature, the imimization degree of PMI increased, which improved the thermal stability of foam. At the same time, the high foaming temperature also increased the size of the bubble, resulting in a decrease in the density and mechanical properties of the foam. When foamed at 200℃, the foam had a low apparent density of 63 kg/m3, and exhibited excellent thermal stability and mechanical properties, with a glass transition temperature (Tg) of 216.1℃ and a compressive strength of 1.08 MPa.