Novel polyetherimide (PEI) dielectrics with cross-linked networks were firstly prepared by utilizing 2,4,6-Triaminopyrimidine (TAP) as cross-linker. The results showed that the breakdown strength of cross-linked PEI with 2 wt% TAP increased to 399.4 MV/m, showing an increment of 23.3% in comparison with non-cross-linked PEI. Additionally, owing to the restrained polarization loss and relaxation loss by cross-linked networks, the dielectric loss of cross-linked PEI dielectric gradually decreased with increasing content of TAP. Of particular significance was that the c-PEI dielectrics exhibited improved discharged energy density (Ud) and charge-discharge efficiency (η) at high temperature. The maximum Ud of cross-linked PEI with 2 wt% TAP was 2.53 J/cm3 at 150 °C, which was 24.8% higher than non-cross-linked PEI. This research provides an innovative strategy to achieve novel PEI dielectrics with improved Ud and η at high temperature.