The Raman gain coefficient is a basic parameter for describing the Raman scattering.The coefficient is usually measured by small signal gain method

for which signal light is necessary.However

the measurement process is long and the measurement result is apt to be affected by unsteady pump light and signal light.In this paper

we put forward a new measurement

i.e.the Raman gain coefficient of optical fibers is obtained by the demodulation of the stimulated Raman spectrum and the spontaneous emission spectrum.The gain coefficient of the conventional G.652 Single Mode Fiber（SMF） is measured and the measurement result is compared with that by the small signal gain method.