GaMnAs grown by low temperature molecular beam epitaxy have been investigated by Fourier transform infrared and photo-modulation reflectance (PR) measurements. Besides the band gap of Ga(Mn)As，Franz-Keldysh oscillations and spin-orbit coupling energy， low energy oscillations， lower than the GaAs band gap， were well observed in the PR spectra of GaMnAs. And these oscillations were also observed clearly in reflectance spectra of GaMnAs. By fitting the oscillations of the reflectance spectra， it was attributed to the variable refractive indexes induced by the high hole density in GaMnAs， which was caused by Mn doping in GaAs. At the same time， the PR spectra of high-resistance GaMnAs at low temperature were also studied.