  We perform an analysis of the $D^+ \to K^0_S \pi^+ \pi^0$ Dalitz plot using a
data set of 2.92 fb$^{-1}$ of $e^+e^-$ collisions at the $\psi(3770)$ mass
accumulated by the BESIII Experiment, in which 166694 candidate events are
selected with a background of 15.1%. The Dalitz plot is found to be
well-represented by a combination of six quasi-two-body decay channels
($K^0_S\rho^+$, $K^0_S\rho(1450)^+$, $\overline{K}^{*0}\pi^+$,
$\overline{K}_0(1430)^0\pi^+$, $\overline{K}(1680)^0\pi^+$,
$\overline{\kappa}^0\pi^+$) plus a small non-resonant component. Using the fit
fractions from this analysis, partial branching ratios are updated with higher
precision than previous measurements.
