Surface ablation significantly affects the distribution of plasma in high-speed flow and the characteristics of its interaction with electromagnetic fields. Considering the mechanism of ablation and ejection on the surface of hypersonic vehicle, the participation of ablation products in the plasma generating process in the flow field, the conduction mechanism of mixed ionized gas containing alkali metal and the electromagnetic dynamics mechanism, the coupled calculation method of high-speed flow/plasma/electromagnetic field with alkali metal ablation is established by solving the three-dimensional thermochemical non-equilibrium flow governing equation with electromagnetic source term, the electric field Poisson equation and the magnetic vector Poisson equation. Combined with the common ablation and pyrolysis process of carbon-carbon materials and silicon-based phenolic resin materials, the mechanism and law of the interaction between surface ablation and electromagnetic field on the hypersonic plasma sheath under various conditions are systematically studied. The results show that the ablation effect affects the plasma distribution in the flow field, which is affected by the ablation mass ejection rate and the mass proportion of alkali metal. When the alkali metal content is high, the alkali metal ionization reaction is dominant, and the electron number density can increase by 1–2 orders of magnitude. The influences of different materials on plasma are different. The mass ejector ratio of silicon-based phenolic resin is larger, and the molar concentration of CO ⁺and C ⁺produced by ionization is close to that of NO ⁺and ${\mathrm{O}}_2^+ $ , which cannot be ignored. Alkali metal in ablative material can significantly improve the control effect of magnetohydrodynamics. With the increase of the proportion of alkali metal, the coupling effect of electromagnetic field increases, and the relationship between them is nonlinear. When the speed is low, the ionization degree of air itself is low and the coupling effect of electromagnetic field is weak. But the efficiency of “improving the electromagnetic effect by ablation of alkali metal” is higher.