Multi-phonon transitions, as a significant electron-phonon coupling phenomenon in solid-state physics, profoundly impact the performance of semiconductor materials and devices. For material systems dominated by deep-level defects, non-radiative multi-phonon transitions represent not only a physical bottleneck limiting the performance of optoelectronic devices but also a necessary mechanism for achieving specific functions. Given this dual role, this article systematically reviews the historical development and current frontiers of the theory on non-radiative multi-phonon transitions. In 1950, Kun Huang and Avril Rhys first established the theory's quantum mechanical framework based on the adiabatic model. After a decades-long debate with the static coupling model, Professor Huang ultimately proved their physical equivalence, laying a solid foundation for the theory's modern development. Entering the 21st century, leveraging the powerful capabilities of first-principles calculations and innovations in computational strategies, precise ab initio methods have revolutionized the field. This review highlights two mainstream methodologies: the all-phonon spectrum method utilizing the combined dynamic matrix (CDM) technique to rigorously solve phonon modes, and the one-dimensional approximation which provides clear physical intuition for systems with strong lattice relaxation. These methods have significantly deepened our physical understanding. For instance, calculations of the V_Ga-O_N center in GaN demonstrate the pivotal role of defect excited states in facilitating efficient recombination cycles, while studies on amorphous SiO₂ reveal a dual-level mechanism driven by metastable configurations. Beyond harmonic approximations, the review also emphasizes the necessity of incorporating anharmonic effects and phonon renormalization (characterized by the Duschinsky matrix) to correct severe deviations in capture cross-sections. Currently, the field is shifting from calculating static transition rates to simulating real-time dynamics. By employing non-adiabatic molecular dynamics (NAMD) based on time-dependent density functional theory (TDDFT), complex interactions involving spin, polarons, excitons, and nuclear quantum effects (NQE) are being integrated into unified models. We examine the application of NAMD in elucidating intricate dynamic processes, including spin-dependent recombination in TiO₂, polaron-accelerated relaxation in FeOOH, ultrafast interlayer exciton transfer in MoSe₂/WSe₂ heterojunctions, and the impact of NQE on carrier lifetimes in lead halide perovskites. These advancements not only provide a lucid physical picture of the complex dynamics of deep-level defects, but also offer robust theoretical guidance for the design and fabrication of high-performance semiconductor devices.