To meet the fast and stable operational demands of DC circuit breakers (DCCBs), an ultrafast mechanical switch (UFMS) based on a Thomson coil (TC) actuator is studied. First, a 2D finite element (FE) transient axisymmetric model of a TC actuator coupled with magnetic fields, solid mechanics and electrical circuit modules is implemented. A time-domain analysis shows that the armature bending may limit the electromagnetic coupling effects between the coil and armature at the initial stage of driving, and this motion is accompanied by the fluctuation of the displacement and stress of each part. A frequency-domain analysis shows that the dynamic response form and vibration frequency of the actuator under the action of repulsive force are consistent with those of the 4th mode, where the maximum relative displacement occurs at the armature edge. Based on this research, a new armature structure is proposed, and the relationships among armature parameters, motion characteristics, and stress distribution are studied. Finally, a 110 kV UFMS is developed, and two armatures with different parameters are built and tested to verify the accuracy of the simulation model and the effectiveness of the optimization method.