In order to improve the link gain and transmission distance of radio-over-fiber(Ro F) frequency transmission link,ultra-long distributed second-order Raman amplifier structure in L band was used to amplify the light signal. The principle of the distributed 2-order Raman amplifier and the Ro F transmission link were explained theoretically. The relationships among them were analyzed by using the coupling equations between signal light,1-order pumping light,2-order pumping light and noise. The 1-order radio f...