  The quantum electrodynamic (QED) theory predicts the photon emission and pair
creation involved in QED cascades occur mainly in a forward cone with finite
angular spread $\Delta\theta \sim 1/\gamma_{i}$ along the momenta of incoming
particles. This finite beaming effect has been assumed to be negligible because
of the particles' ultra-relativistic Lorentz factor $\gamma_{i}\gg1$ in
laser-driven QED cascades. We develop an energy- and angularly resolved
particle-tracking code, resolving both the energy spectra and the momentum
profile of the outgoing particles in each QED event, which improves
substantially the agreement between the simulation and exact QED results. We
investigate QED cascades driven by two counter-propagating circularly polarized
laser pulses, and show that the narrow beaming could be accumulated to
effectively suppress the long-term growth of cascades, even though it can
hardly affect the early formation of cascades. For QED cascades longer than
$10$ laser cycles, the finite beaming effect could decrease the final pair
yield, especially at ultrahigh intensities $\xi>600$, by more than $10\%$.
