  The primordial black hole (PBH) is an effective candidate for dark matter. In
this work, the PBH abundance $f$ is calculated in peak theory, with one or two
perturbations in the inflaton potential. We construct an antisymmetric
perturbation that can create a perfect plateau in the inflaton potential,
leading inflation to the ultra-slow-roll stage. During this stage, the power
spectrum of primordial curvature perturbation is remarkably enhanced on small
scales, generating abundant PBHs. The PBH abundance $f\sim 0.1$ can be achieved
in one or two typical mass windows at $10^{-17}M_\odot$, $10^{-13}M_\odot$, and
$30M_\odot$, without spoiling the nearly scale-invariant power spectrum on
large scales. For comparison, $f$ is calculated in two approximate methods of
peak theory (with different spectral moments) and also in the Press--Schechter
theory. It is found that the Press--Schechter theory systematically
underestimates $f$ by two or three orders of magnitude compared with peak
theory.
