  MAXI J1816-195 is an accreting millisecond X-ray pulsar (AMXP) discovered in
2022. According to the Insight-HXMT data, the pulsations of this source extend
all the way to over 100 keV, and its pulse profiles change from a single peak
in low-energy range to double peaks in high-energy range. In this work, we
simulate its energy spectra and pulse profiles with a Compton scattering Monte
Carlo program. The simulation results suggest that the low energy X-ray source
on the neutron star surface should be pencil-beamed radiations from the
magnetic poles, and there should be a boundary layer in a hollow cylinder shape
between the accretion disc and the neutron star surface: the up-scattering of
the polar radiations in the boundary layer leads to the double peak structure
of the high-energy pulse profile. Under this boundary layer geometry, we
suggest that the rarity of AMXPs can be caused by the smearing of the boundary
layer. To estimate the mass M and radius R of accretion-powered millisecond
pulsars whose surface radiations are badly polluted by the accretion disk and
boundary layer, the impact of Compton scattering in the boundary layer on the
radiation should be removed before employing the X-ray pulse profile modeling
method.
