The effect of pumping light linewidth on fiber Raman amplifier used in density wavelength division multiplexed (DWDM) system is analyzed by applying SRS coupling wave theory.It is pointed out that the pumping efficiency will be reduced if the wide-spectrum pumping light is used.The variation curves of pumping light linewidth and power versus pumping efficiency are given.A conclusion is presented that lower pumping efficiency will be obtained with larger pumping light power,pumping light linewidth, Raman gain and lesser effective fiber section.