Abstract: In order to improve the link gain and transmission distance of radio-over-fiber (RoF) 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 RoF 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 frequency (RF) signal gain of a RoF transmission system based on ultra-long distributed 2-order Raman amplifier was also obtained. Through numerical simulation and system experiment, the effect of pumping power on on-off gain of ultra-long distributed 2-order Raman amplifier, frequency response and RF gain of RoF transmission system in the range of 0GHz~7GHz, and phase noise of RoF transmission link after the application of ultra-long distributed 2-order Raman amplifier were obtained. The results show that the link gain of 28.1dB is obtained by RoF transmission after the application of an ultra-long distributed 2-order Raman amplifier. Approximate lossless transmission is achieved in optical link with a distance of 80.94km. The on-off gain of RF signal is independent of the frequency of RF signal. The study has important application value in long distance transmission of optical radio frequency links.