  Jet precession is considered to universally exist in different-scale
astronomical phenomena, including gamma-ray bursts (GRBs). For the long-lived
GRB central engine, the relativistical precessing jets will periodically inject
kinetic energy into the external shocks, then significantly modulate the shapes
of the light curves (LCs) in GRB afterglows. In this paper, we adopt the
standard external shock model to investigate the effects of jet precession on
GRB X-ray afterglows in cases with different parameters, i.e., the steady or
time-dependent jet powers, precession periods, precession angles, and viewing
angles. In the case where the jet powers are in steady or slow decay and the
jet can sweep across the line of sight, shallow decay (or plateau) segments
should appear; otherwise, a giant bump will emerge in the GRB afterglow LCs. We
show that jet precession is a new plausible mechanism of the energy injection
in GRBs. Moreover, some observed X-ray transients without GRB associations
might be powered by the precessing jets.
