The evolution of the Kelvin-Helmholtz (KH) instability in the presence of classical hydrodynamics and magneto-hydro-dynamics is investigated numerically by using the magneto-hydro-dynamic (MHD) equations. The MHD equations are solved with the corner transport upwind plus constrained transport algorithm that guarantees the divergence-free constraint in the magnetic field. The numerical results are used to analyze the effects of magnetic field ( ${M_{\rm{A}}} = 3.33$ ) on the vorticity and pressure evolution of mixing layer, and also compared with those in the hydrodynamics situation. Moreover, the mechanism of weakening the effect of magnetic field on the KH instability is revealed from the perspectives of the magnetic pressure and the magnetic tension. The results show that the external magnetic field has a great influence on the flow structure of the mixing layer. Specifically, the magnetic pressure has a major effect in the vorticity deposition on the interface, whereas the magnetic tension generates a torque to counter the scrolling of vortex. As a result, the large vortex structure is stretched and destroyed, and finally restrains the vortex rolling-up. In addition, with the development of mixing layer, the interface will separate at the points of maximum curvature under the joint effect of the magnetic pressure, the magnetic tension and the pressure field, and finally form a fishhook-like vortex structure.