  We show that the Pomeron exchanges play a unique role in vector charmonium
scatterings. Such a mechanism can provide a natural explanation for the
nontrivial structures in the di-$J/\psi$ spectrum observed by the LHCb
Collaboration. The narrow structure $X(6900)$, as a dynamically generated
resonance pole, can arise from the Pomeron exchanges and coupled-channel
effects between the $J/\psi$-$J/\psi$, $J/\psi$-$\psi(2S)$ scatterings. A pole
structure near the di-$J/\psi$ threshold is also found. Meanwhile, we predict
that $X(6900)$ can produce significant threshold enhancement in the
$J/\psi$-$\psi(2S)$ energy spectrum which can be searched for at LHCb.
