We report a long wave cutoff perfect absorber (LWCPA) with a nanocone array structure. Each nanocone unit comprises an InP cone grown on a SiO₂ substrate, with three layers of stacked structures nested inside: from bottom to top, these are Si₃N₄ blocks, Cr blocks, and Au cones. The structure exhibits outstanding absorption performance from ultraviolet to near-infrared wavelengths, achieving an average absorption of 97.9% in the 200–820 nm band. A sharp transition occurs as absorption drops dramatically from 95% at 820 nm to 10% at 1880 nm, with an average absorption of only 3.5% in the non-absorption band of 1880–4000 nm. Key performance metrics include an extinction ratio of 9.78 dB, an extinction difference of 85%, and a cutoff slope of 0.234 nm⁻¹. Considering that this structure may be applied to photoelectric conversion systems in the future, the present invention incorporates Si₃N₄ in advance and verifies that the absorption rate remains at a high level under its influence, thereby laying the groundwork for subsequent application of this structure in photoelectric conversion systems. The design was optimized via finite-difference time-domain (FDTD) simulations, with structural parameters such as nanocone height and base dimensions fine-tuned to enhance absorption and cutoff characteristics. This LWCPA offers an efficient strategy for ideal solar heat absorber design, with significant potential for applications in renewable energy technologies, including solar thermal photoelectric and solar thermal energy systems.