The burgeoning growth of the new energy vehicles and aviation industry has escalated the need for energy storage capacitors capable of stable operation in harsh environments. The advent of metal-polyimide complexes has illuminated a novel approach for preparing temperature-resistant capacitors. However, the general application of these metal-polyimide complexes is impeded by the high dielectric loss and low breakdown strength, consequences of main-chain coordination and excessive metal ions content. Herein, our study proposes a novel polyimide-Cu complex material (POP-Cu) predicated on side-chain-type pyridine-Cu coordination, utilizing the structural backbone PMDA-ODA of mature commercial PI (Kapton) with reliable performance. Owing to the high degree of freedom afforded by the side chain with suppressed relaxation activation energy and the long-range electron delocalization formed by d-π coordination, the dielectric constant of this material containing merely 2.7 mol% Cu increases from 3.25 (POPI) to 5.58, while maintaining a remarkably low dielectric loss of 0.0066. Meanwhile, this material exhibits a substantial DC breakdown strength of 436.2 MV·m−1 and a high energy density of 5.42 J·cm−3, coupled with superior mechanical and thermal properties. Even at 150 °C, it retains over 90% of its room-temperature energy density, demonstrating notable dielectric stability under high temperatures. These attributes underscore its promising application for capacitors operating in harsh environments.