Semi analysis expressions of various electrical noise terms in distributed fiber Raman amplifiers are derived in this paper, accounting to impacts of Rayleigh backscattering on amplified spontaneous emission (ASE) and double Rayleigh backscattering (DRB) noise. The concept of equivalent sensitivity is introduced in this paper. The impacts of Rayleigh backscattering on the noise performance of distributed Raman amplifier is studied comprehensively by this concept.