1.70 μm band laser sources have larger applications in many fields, such as biomedical medicine, infrared light generation and so on; the gain spectrum in 1.70 μm band is an international hotspot. A home-made multimode fiber laser is applied as the Raman pump in order to avoid stimulated Brillouin scattering. The pumped signal is injected into the high nonlinear fiber (HNLF) and dispersion shifted fiber (DSF) through 1550 nm/1650 nm wavelength division multiplex (WDM). Firstly, the principle of generating gain spectrum using the multimode laser is analyzed. Then, the forward and backward spectra are analyzed, these spectra are produced by HNLF and DSF with different lengths under the condition of different powers. The results show that, 1 km long HNLF and 6 km long DSF can be used to obtain the best backward gain spectrum with maximum power and peak wavelength of near 1700 nm. The experimental results provide technical reference for 1.70 μm band continuous wave lasers and ultrafast lasers.