The diffusion couple technique is a valuable experimental methodology for investigating phase relations within multicomponent systems. Demonstrating significantly greater efficiency compared to conventional approaches, this method enables the acquisition of crucial materials data that would otherwise be excessively time-consuming and costly, and these obtained data can be leveraged to expedite alloy design processes. Essential information for structural materials design, such as phase diagrams, diffusion coefficients, precipitation kinetics, solution-strengthening effects, and precipitation-strengthening effects, can be efficiently obtained through this approach. In this study, diffusion couple was employed to determine the phase diagram of the Cr-Mo-Si ternary system was prepared by encapsulating the diffusion segments in Ni shell. The introduction of Cr to Mo-Si-based alloys is known to generate a protective Cr2O3 layer and α-cristobalite layer, thereby enhancing resistance to high-temperature oxidation. The isothermal sections at 1200 ℃ and 1300 ℃ and the liquidus projection of the Cr-Si-Mo ternary system have been experimentally determined based on the microstructures, the phase structure analyses and the phase constitutions of the as-cast and isothermally heat-treated alloys in an entire composition range using microhardness testing and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD).