A viscous warm pressure forming method is proposed to reduce the cost in small batch for complex shape thin-walled parts of polyetherimide (PEI), which is based on the mechanical properties of PEI and combines with the advantages of viscous pressure forming method. The tensile test of PEI sheet, warm property experiment of viscous medium, and viscous warm pressure bulging experiment of PEI sheet were carried out to preliminary investigate the forming feasibility, deformation law, and forming quality. The results show that a PEI sheet is strongly sensitive to forming temperature and loading speed. From 20 to 150 °C, with the increasing of temperature and decreasing of loading speed, the formability is enhanced gradually. Furthermore, the formability of PEI sheet is greater in biaxial stress state than that in uniaxial stress state. The forming quality, surface finish, and light transmittance are not influenced by the deformation of PEI sheet. Meanwhile, different from the wall thickness distribution of sheet metal bulging specimen, the maximum wall thickness reduction rate of PEI sheet presents the regional distribution which locates at the center of the bulging specimen, and with the increasing of temperature and deformation, the area of regional distribution increases.