  BaNi$_2$As$_2$ exhibits a first order structural transition at 130 K.
Understanding this structural transition is a crucial step towards
understanding the electronic properties of the material. We present a combined
optical spectroscopy and band structure calculation study on the compound
across the transition. The study reveals that BaNi$_2$As$_2$ is a good metal
with a rather high plasma frequency. The phase transition leads to a small
reduction of conducting carriers. We identify that this reduction is caused by
the removal of several small Fermi surface sheets contributed dominantly from
the As-As bonding and Ni-As antibonding states.
