  Diffuse gamma-ray emission (DGE) has been discovered over the Galactic disk
in the energy range from sub-GeV to sub-PeV. While it is believed to be
dominated by the pionic emission of cosmic ray (CR) hadrons via interactions
with interstellar medium, unresolved gamma-ray sources may also be potential
contributors. TeV gamma-ray halos around middle-aged pulsars have been proposed
as such sources. Their contribution to DGE, however, highly depends on the
injection rate of electrons and the injection spectral shape, which are not
well determined based on current observations. The measured fluxes of DGE can
thus provide constraints on the $e^\pm$ injection of the pulsar halo population
in turn. In this paper, we estimate the contribution of pulsar halos to DGE
based on the ATNF pulsar samples with taking into account the off-beamed
pulsars. The recent measurement on DGE by Tibet AS$\gamma$ and an early
measurement by MILAGRO are used to constrain the pair injection parameters of
the pulsar halo population. Our result may be used to distinguish different
models for pulsar halos.
