The far infrared reflection spectra of n GaAs_(1-x)_P_x_ with x=0.328 and x=0.40 are investigated in the frequency range of 40-700 1/cm and the temperature range of 4.2-300K. The important physical parameters describing the optical phonons, Plasmon and LO phononplasmon coupling mode of the material and infrared optical constants are obtained by the multioscillator fitting and K-K relation calculation. A model of two conduction band valley with a donor level below and associated with X minimum is present to explain the behavior of the dependence of plasmon frequency temperature on temperature and the variation of the three coupling modes with temperature.