  Synchrotron emission polarization is very sensitive to the magnetic field
configuration. Recently, polarization of synchrotron emission with a mixed (SM)
magnetic field in Gamma-ray burst (GRB) afterglow phase had been developed.
Here, we apply these SM models to GRB prompt phase and compare their
polarization properties with that of synchrotron emission in purely ordered
(SO) magnetic field. We find that the polarization properties in a SM model are
very similar to these in a corresponding SO model (e.g., synchrotron emission
in a mixed magnetic field with an aligned ordered part (SMA) and synchrotron
emission with a purely ordered aligned magnetic field (SOA)), only with a lower
polarization degree (PD). We also discuss the statistical properties of the
models. We find PDs of the simulated bursts are concentrated around $25\%$ for
both SOA and synchrotron emission in a purely ordered toroidal magnetic field
(SOT), while they can range from $0\%$ to $25\%$ for SMA and synchrotron
emission in a mixed magnetic field with a toroidal ordered part (SMT),
depending on $\xi_B$ value, i.e., the ratio of magnetic reduction of the
ordered magnetic field over that of random magnetic field. From statistics, if
PDs of majority GRBs are non-zero, then it favours SO and SM models. Further,
if there are some bright GRBs with a prominently lower PDs than that of the
majority GRBs, it favours SOT (SMT) models; if all the bright GRBs have
comparable PDs with the majority ones, it favours SOA (SMA) models. Finally, we
apply our results to POLAR's data and find that $\sim10\%$ time-integrated PDs
of the observed bursts favor SMA and SMT models, and $\xi_B$ parameter of these
bursts is constrained to be around 1.135.
