Mo and its alloys exhibit considerable potential for aerospace high-temperature components, electronic thermal management systems, and high-temperature power-generation structures due to their high melting point, excellent elevated-temperature mechanical strength, and good creep resistance. However, their application is severely limited by rapid oxidation at temperatures above 700 ^oC, where the formation and volatilization of MoO₃ lead to accelerated material loss and structural degradation. This oxidation susceptibility can ultimately result in disintegration and catastrophic failure under extreme service conditions. The application of silicide-based coatings is an effective strategy to mitigate high-temperature oxidation by forming a protective barrier that isolates the substrate from the environment. Nevertheless, monolithic silicide coatings often suffer from premature failure caused by thermal expansion mismatch with the substrate and inward silicon diffusion during prolonged high-temperature exposure. In this context, silicide-boride composite coatings have emerged as a promising alternative for further improving oxidation resistance. Despite their potential, the mechanisms governing gradient microstructure formation and the origins of performance variability in such composite coatings remain insufficiently understood. In this study, silicide and silicide-boride composite coatings were fabricated on pure Mo substrates using halide-activated pack cementation, and their microstructural evolution and high-temperature oxidation behavior were systematically investigated. The results demonstrate that B element incorporation promotes the formation of a silicide-boride composite coating with a five-layer graded structure: MoSi₂/(MoSi₂ + MoB)/Mo₅Si₃/MoB/Mo₂B. Notably, B facilitates the preferential formation of an initial MoB interlayer at the coating-substrate interface. This interlayer not only inhibits the directional diffusion of Si but also induces a displacement reaction between Si and MoB to form MoSi₂, thereby suppressing the (001) preferred growth orientation of MoSi₂. In addition, volume contraction associated with MoB formation within the MoSi₂ + MoB mixed layer generates pores and a roughened interface, which act as high-density nucleation sites and significantly refine the surface MoSi₂ grain structure. The refined grain structure accelerated the formation of a dense and continuous SiO₂ protective film, thereby effectively inhibiting O diffusion. After 30 h of oxidation at 1200 ^oC, the silicide-boride composite coating exhibited an oxidation mass gain of 1.28 mg/cm² and an oxidation rate constant of 0.29 mg/(cm²·h), representing a 53% reduction relative to the silicide coating. Moreover, the MoB interlayer suppressed inward Si diffusion into the substrate, thereby enhancing long-term stability under high-temperature oxidative conditions.