We calculate here using the mean field theory, the phase-line equations for the gamma-beta, delta-beta and delta-gamma phase transitions in the NH4BrxCl1-x system. We fit our P-T phase diagrams calculated at various concentrations to the experimental data for this crystalline system. Our results show that there is a good agreement between our calculated and experimentally observed phase diagrams for the NH4BrxCl1-x system.