In a forward multi-longitudinal mode laser diode(LD)-pumped Raman fiber amplifier,the comb-like waves with more than 100 wavelengths are found on both sides of the amplified signal wavelength. The generation mechanism of the comb-like waves is analyzed and investigated. The four-wave mixing between a number of longitudinal modes of pump LD and the input signal is the fundamental reason that the comb-like waves generate. The effects of the pump power,signal power,signal wavelength and pump configuration on the comb-wave spectra are observed and investigated.