In this paper, we performed magnetohydrodynamics numerical experiments to look into the dynamic behaviour of the current sheet (CS) between the coronal mass ejection (CME) and the associated solar flare, especially the CS oscillation and plasmoid motions in coronal conditions. During the evolution, the disrupting magnetic configuration becomes asymmetric first in the buffer region at the bottom of the CME bubble. The Rayleigh−Taylor instability in the buffer region and the deflected motion of the plasma driven by the termination shock at the bottom of the CME bubble cause the buffer region to oscillate around the y-axis. The local oscillation propagates downwards through the CS, prompting an overall CS oscillation. As the buffer region grows, the oscillation period becomes longer, increasing from about 30 s to about 16 min. Meanwhile, there is another separated oscillation with a period between 0.25 and 1.5min in the cusp region of the flare generated by velocity shearing. The tearing mode instability yields formations of plasmoids inside the CS. The motions of all the plasmoids observed in the experiment accelerate, which implies that the large-scale CME/flare CS itself in the true eruptive event is filled with the diffusion region according the the standard theory of magnetic reconnection.