The encapsulation of hybrid perovskite in nanoporous GaN is a promising strategy to reduce perovskite degradation and improve photoelectric performance. Exploitation of the intrinsic light-matter interaction in such a composite structure greatly benefits improving device optimization. Herein, we report the perovskite synthesis, structural design, and radiative properties of perovskite/nanoporous GaN hierarchical composites using spectroscopic ellipsometry (SE) experiments as well as electromagnetic simulations. First-principles calculations reveal the relationship between electronic interband transitions and the dielectric function of perovskite measured by SE. We use the finite-difference time-domain (FDTD) and finite-element method (FEM) simulations to analyze the spectral radiation properties for the hierarchical composite structure in terms of both morphology dependence and material factors. The optical absorption peaks in the near-infrared–visible region are collectively determined by the nanopore size, layer thickness, and incident angle of light. Also, the optical absorption distribution patterns are related to variables of the halogen, metal elements, and temperature. This work is integrated with the development of perovskite-based applications for solar functional batteries, light-emitting diodes (LED), and optoelectronic detectors.