LaMnO₃ perovskite has great potential in microwave absorption at high temperature due to its complex doping effect and super stability. The current research mainly focuses on the doping ratio regulation, while the mechanism of doping effect at high temperature is still lack of sufficient investigation. In this work, La_1−xSr_xMn_1−yFe_yO₃ (LaMnO₃, La_0.7Sr_0.3MnO₃, and La_0.7Sr_0.3Mn_0.8Fe_0.2O₃) nanostructures with different doping sites were successfully prepared by the solid phase reaction method. Then, the high temperature dielectric test samples were obtained by mixing with cordierite (2MgO·2Al₂O₃·5SiO₂ (MAS)). The results showed that the temperature dependence of Mn ion spin state had a significant impact on the high temperature dielectric behavior of La_1−xSr_xMn_1−yFe_yO₃. Particularly, when the thickness is only 1.9 mm, La_0.7Sr_0.3Mn_0.8Fe_0.2O₃/MAS can achieve the widest bandwidth of 4.2 GHz covered the entire X-band (8.2–12.4 GHz) and a minimum reflection loss (RL) value of −17.99 dB at 500 °C. In order to improve the operating temperature of La_0.7Sr_0.3Mn_0.8Fe_0.2O₃/MAS, a cellular array structure was designed by using computer simulation technology (CST) software to introduce magnetic loss. When the outer length of the hexagon is 1 mm and the coating thickness is 1.9 mm, the widest bandwidth covers the X-band and the minimum RL value is −15.35 dB at 800 °C. Therefore, La_0.7Sr_0.3Mn_0.8Fe_0.2O₃ has a great prospect as an efficient high temperature microwave absorber.