Abstract: As a quintessential representative of ternary hard protective coatings, CrAlN coatings are high-performance materials developed by alloying CrN coatings with aluminium. They exhibit excellent comprehensive properties, including high hardness, low friction coefficient, outstanding high-temperature oxidation resistance, and corrosion resistance. Fabricated via mainstream processes such as physical vapour deposition (PVD), CrAlN coatings have been widely applied in extreme service scenarios, such as cutting tool manufacturing, aero-engine components, marine engineering equipment, and nuclear industry protection. This paper systematically reviews the microstructural characteristics and regulation mechanisms of CrAlN coatings. The core findings are as follows: aluminium content is a key parameter governing the structure and properties of the coatings. It stabilizes a single face-centred cubic (FCC) phase within a critical threshold (atomic fraction < 0.71), while a transition to dual-phase or hexagonal phases occurs when aluminium content exceeds this limit. Multilayer structural design and doping with elements such as Si, Y, and B significantly enhance the coating performance—including a maximum hardness exceeding 38 GPa and a high-temperature oxidation resistance threshold of up to 900 ℃, via interface strengthening, solid solution strengthening, and grain refinement effects, thereby establishing clear microstructure–macro property correlations. Current research faces challenges such as insufficient interfacial stability and limited long-term high-temperature protection capability. Future work should focus on process innovation (e.g., composite PVD techniques), optimization of multi-component nano-multilayer/gradient structures, and theoretical and experimental verification of multi-element synergistic doping. Meanwhile, expanding the application of CrAlN coatings in emerging extreme operating conditions will provide crucial support for their theoretical advancement and engineering implementation.