  Polarization can serve as a probe of the radiation mechanism and magnetic
field (MF) configuration in gamma-ray bursts (GRBs). In the case of constant
MF, the synchrotron polarization in the prompt phase of GRBs has been widely
studied. In this paper, we consider the case of the decaying MF. We calculate
the time-averaged and instantaneous synchrotron polarizations in a pulse for
different viewing angles and for the large-scale decaying MF model, which can
explain the so-called Band spectrum. We find that the on-axis time-averaged
polarization degree (PD) in the energy band of 50-500 keV for the decaying
large-scale MF model ($\sim 0.6$ for typical parameters) is higher than that in
the constant MF model ($\sim 0.5$). An interesting result is the instantaneous
PD in the off-axis case will experience a turnover, i.e., the PD will evolve
from a positive value to a negative one. This suggests the polarization angle
(PA) change by an angle of $90^\circ$. Such a result is roughly consistent with
the discovery of the PA evolution within a pulse in some bursts, such as GRB
170114A and GRB 160821A. Our result implies at least a part of bursts (off-axis
bursts) should have the PA evolution in a pulse.
