  We study the anomalous cross-section lineshape of $e^+e^-\rightarrow D\bar D$
with an effective field theory. Near the threshold, most of the $D\bar D$ pairs
are from the decay of $\psi(3770)$. Taking into account the fact that the
nonresonance background is dominated by the $\psi(2S)$ transition, the produced
$D\bar D$ pair can undergo final-state interactions before the pair is
detected. We propose an effective field theory for the low-energy $D\bar D$
interactions to describe these final-state interactions and find that the
anomalous lineshape of the $D\bar D$ cross section observed by the BESII
collaboration can be well described.
