Abstract: The Quantum Cascade Laser (QCL), based on the subband transition mechanism, exhibits wide wavelength tunability and high-power efficiency. It serves as a core light source in the mid- and far -infrared (3–25 μm) to terahertz (1–6 THz) spectral bands, offering irreplaceable advantages in trace gas detection, biomedical imaging, and communication. This paper provides a systematic review of the physical mechanisms, historical development, and material structure advancements of QCLs, with a focus on the design progress of multi-band devices. Initially, the regulation mechanisms of interband transitions and cascade effects are analyzed. Subsequently, design strategies for the active region, such as double phonon resonance and bound-to-continuum transitions, are described in detail. Optical and thermal simulations of single and double active regions reveal that the insertion of an InP spacer layer in dual active regions can significantly enhance heat dissipation performance. Based on these findings, the pathways to achieving high-power (>1 W) and high-efficiency (>15 %) devices are summarized, along with multi-band application results. These include advancements in mid-infrared gas sensing systems (with detection limits reaching ppt levels) and terahertz imaging (resolution of 200 μm). The review concludes by identifying future technical challenges, which include optimizing thermal management for continuous operation in the mid- to far-infrared bands, achieving room-temperature operation in the terahertz range, and improving power output.