In this paper, the magnetic properties of Heusler alloy Mn₂CoAl bulk and the atomic relaxation, magnetism, electronic structure and surface atomic polarization behavior of Mn₂CoAl (100) surface were systematically studied by first-principles calculations. Atomic relaxation calculations reveal that the MnAl termination exhibits interlayer differential displacement characteristics: Mn atoms in the first layer relax inward toward the termination while Al atoms shift outward toward the vacuum layer, atoms in the second layer collectively migrate toward the vacuum layer, and those in the third layer display an inward contraction tendency. The remaining three terminations, MnCo, MnMn and CoCo, follow similar patterns, with all three atomic layers in each termination showing slight outward displacements toward the vacuum layer. Notably, no significant deformation occurs in the overall structures of all four terminations. The half-metallic gaps at the MnCo and CoCo terminations of Mn₂CoAl(100) are destroyed by the surface state, while the MnAl and MnMn terminations are not significantly affected and have larger half-metallic gaps, retaining about 92% and 80% of the polarization, respectively, which is predicted to have a superior potential for possible applications in tunnel junctions.