  The electronic and phonon properties of the platinum pnictide superconductors
$A$Pt$_3$P ($A$=Ca, Sr, and La) were studied using first-principles
calculations. The spin-orbit coupling effect is significant in LaPt$_3$P but
negligible in CaPt$_3$P and SrPt$_3$P, although they all share the same
anti-pevroskite structure. Moreover, SrPt$_3$P has been demonstrated to exhibit
an unexpected weak charge-density-wave(CDW) instability which is neither simply
related to the Fermi-surface nesting nor to the momentum-dependent
electron-phonon coupling alone. The instability is absent in CaPt$_3$P and can
be quickly suppressed by the external pressure, accompanied with gradual
decreases in the phonon softening and BCS $T_c$. Our results suggest SrPt$_3$P
as a rare example where superconductivity is enhanced by the CDW fluctuations.
