The photoluminescence of GaAs0.15P0.85: N has been investigated under hydrostatic pressure at 77 K. The NN1 emission is clearly seen when P>10kbar. Meanwhile, luminescence quenching and band narrowing of Nx line have been observed under pressure. The results show that the pressure effectively enhances the Nx→NN1 thermally assisted exciton transfer processes. The pressure behaviors of Nx and NN1 levels have been analysed and fitted to a theoretical model. The pressure coefficients of the levels and some parameters related to their wavefunctions have been determined by fitting calculations.