  The particle diffusion coefficients of three TeV pulsar halos observed so far
are inferred to be significantly smaller than the typical value of the
interstellar medium (ISM). The anisotropic diffusion model ascribes the slow
diffusion to the cross-field diffusion assuming sub-Alfv{\'e}nic turbulence in
the ISM around the pulsar if the viewing angle between the observer's
line-of-sight (LOS) to the pulsar and the local mean field direction is small.
In general, the TeV halo's morphology under this model highly depends on the
viewing angle, and an elongated, asymmetric morphology is predicted if the LOS
is not approximately aligned with the local mean field direction. While the
specific requirement of a small viewing angle is supposedly established only
for a small fraction of TeV halos, TeV halos with apparent asymmetric
morphology has not been detected. In this paper we will study the expectation
of TeV halos measured by the TeV-PeV gamma-ray detector LHAASO in the framework
of anisotropic diffusion model, with a particular focus on the influence of the
viewing angle on the detectability. We show that a TeV halo is more detectable
with a smaller viewing angle and this selection effect may explain why the
morphologies of all three detected TeV halos so far are consistent being
spherical. We also demonstrate that LHAASO is capable of detecting asymmetric
TeV halos after several-year operation with reasonable source parameters. This
can serve as a critical test of the anisotropic diffusion model.
