  The scalar induced gravitational waves (SIGWs) is a useful tool to probe the
physics in the early universe. To study inflationary models with this tool, we
need to know how the waveform of SIGWs is related to the shape of the scalar
power spectrum. We propose two parameterizations to approximate the scalar
power spectrum with either a sharp or a broad spike at small scales, and then
use these two parameterizations to study the relation between the shapes of
$\Omega_{GW}$ and the scalar power spectrum. We find that the waveform of SIGWs
has a similar shape to the power spectrum. Away from the peak of the spike, the
frequency relation $\Omega_{GW}(k)\sim \mathcal{P}_\zeta^2(k)$ holds
independent of the functional form of the scalar power spectrum. We also give a
physical explanation for this general relationship. The general relation is
useful for determining the scalar power spectrum and probing inflationary
physics with the waveform of SIGWs.
