A new method was developed for measuring the fiber Raman gain spectrums,which is obtained by demodulation of the stimulated Raman spectrum and the spontaneous emission spectrum,this method is proved to have a good performance in the measurement of the fiber Raman gain spectrums,and three wavelength(1428nm，1445nm，1467nm）pump broadband Raman fiber amplifier with gain band over 34 nm(1dB),on-off gain of 13 dB is designed by this method,and the experimental result is agree with the measurement by the small signal gain method.The result of experiment show the way possesses simple performance,high accuracy,high good real-time characteristic and useful value.关