This paper describes the photo luminescence detection apparatus, which consists of a pulsed CuBr laser with low duty cycle and a boxcar averager. It features high sensitivity and good repeatability. The photoluminesoent spectra of a liquid-phase epitaxial GaP:N wafer have been measured in the temperature range 80-300K and for different thicknesses. Based on the spectra, the green-light emitting mechanism is analysed. It is found that the spectra are mainly composed of A-O and NN1 peaks in the range of 80~160 K，while the A peak and its phonon-associated peaks dominate over the spectral in the range of 160~300 K. The NN1 peaks are very prominent in the spectra of different epitaxial layers. Analysis of NN3 peaks could help prepare high efficient green-light-emitting GaP:N materials.