  Based on the dynamics of single scalar field slow-roll inflation and the
theory of reheating, we investigate the generalized natural inflationary (GNI)
model. Concretely, we give constraints on the scalar spectral index $n_{s}$ and
tensor-to scalar ratio $r$ for $\Lambda$CDM $+r$ model according to the latest
data from Plack 2018 TT,TE,EE+lowE+lensing (P18) and BICEP2/Keck 2015 season
(BK15), i.e., $n_{s}=0.9659\pm0.0044$ at $68\%$ confidence level (CL) and
$r<0.0623$ at $95\%$CL. We find that the GNI model is favored by P18 plus BK15
in the ranges of $\log_{10}(f/M_{p})=0.62^{+0.17}_{-0.18}$ and
$m=0.35^{+0.13}_{-0.23}$ at $68\%$CL. In addition, the corresponding
predictions of the general and two-phase reheating are respectively discussed.
It follows that the parameter $m$ has the significant effect on the model
behaviors.
