Multielement carbonitrides have emerged as promising candidates for ultrahigh-temperature thermal protection, yet the influence of compositional variation on their ablation behavior remains insufficiently understood. Here, a series of Hf–Zr–Ti–C–N bulk ceramics with systematically varied metallic atomic ratios are fabricated by spark plasma sintering. A comprehensive evaluation of fracture toughness, flexural strength and oxyacetylene flame ablation performance identifies Hf_4/6Zr_1/6Ti_1/6CN as the optimal ceramic matrix, which is subsequently applied as a coating on graphite with a SiC-graded functional transition layer. After ablation for 120 s at 3000 °C, the coating exhibits excellent ablation resistance, with mass and linear ablation rates of −1.23±0.25 mg/s and −2.07±0.37 μm/s, respectively. Its superior ablation resistance originates from a synergistic interplay of phase transformation into a dense HfO₂-dominated oxide barrier, the self-healing effect of the SiO₂-rich phase coupled with stress relaxation by the Si–C–O nanowire network, and interfacial strengthening induced by the Si-enriched diffusion layer. Combined with molecular dynamics simulations, the oxidation evolution pathway during the initial stage of ablation is further elucidated and validated at the atomic scale. This work establishes a composition-optimized carbonitride coating with outstanding ablation resistance and provides a mechanistic framework for the rational design of next-generation thermal protection systems intended for extreme aerospace environments.