  GRB 160325A is the only bright burst detected by AstroSat CZT Imager in its
primary field of view to date. In this work, we present the spectral and
polarimetric analysis of the prompt emission of the burst using AstroSat, Fermi
and Niel Gehrels Swift observations. The prompt emission consists of two
distinct emission episodes separated by a few seconds of quiescent/ mild
activity period. The first emission episode shows a thermal component as well
as a low polarisation fraction of $PF < 37\, \%$ at $1.5\, \sigma$ confidence
level. On the other hand, the second emission episode shows a non-thermal
spectrum and is found to be highly polarised with $PF > 43\, \%$ at $1.5
\sigma$ confidence level. We also study the afterglow properties of the jet
using {\it Swift}/XRT data. The observed jet break suggests that the jet is
pointed towards the observer and has an opening angle of $1.2^{\circ}$ for an
assumed redshift, $z = 2$. With composite modelling of polarisation, spectrum
of the prompt emission and the afterglow, we infer that the first episode of
emission originates from the photosphere with localised dissipation happening
below it, and the second from the optically thin region above the photosphere.
The photospheric emission is generated mainly by inverse Compton scattering,
whereas the emission in the optically thin region is produced by the
synchrotron process. The low radiation efficiency of the burst suggests that
the outflow remains baryonic dominated throughout the burst duration with only
a subdominant Poynting flux component, and the kinetic energy of the jet is
likely dissipated via internal shocks which evolves from an optically thick to
optically thin environment within the jet.
