We study the radio spectrum of PSR B1259−63 orbiting around the Be star LS 2883 and show that the shape of the spectrum depends on the orbital phase. At frequencies below 3 GHz, PSR B1259−63 flux densities are lower when measured near the periastron passage than those measured far from periastron. We suggest that an interaction of the radio waves with the Be star environment accounts for this effect. While it is quite natural to explain the pulsar eclipse by the presence of an equatorial disc around LS 2883, this disc alone cannot be responsible for the observed spectral evolution of PSR B1259−63 and we, therefore, propose a qualitative model which explains this evolution. We consider two mechanisms that might influence the observed radio emission: free–free absorption and cyclotron resonance. We believe that this binary system can hold the clue to the understanding of gigahertz-peaked spectra of pulsars.