  We present the results of modelling of the radio spectrum evolution and
dispersion measure variations of PSR B1259-63, a pulsar in a binary system with
Be star LS 2883.
  We base our model on a hypothesis that the observed variations of the
spectrum are caused by thermal free-free absorption occurring in the pulsar
surroundings. We reproduce the observed pulsar spectral shapes in order to
examine the influence of the stellar wind of LS 2883 and the equatorial disc on
the pulsar's radiation.
  The simulations of the pulsar's radio emission and its consequent free-free
absorption give us an insight into the impact of stellar wind and equatorial
disc of LS 2883 has on the shapes of PSR B1259-63 radio spectra, providing an
evidence for the connection between gigahertz-peaked spectra phenomenon and the
close environment of the pulsar. Additionally, we supplement our model with an
external absorbing medium, which results in a good agreement between simulated
and observational data.
