A new saturation mechanism, beam plasma instability with trapped electrons resulting in saturation of stimulated Raman backscattering (SRBS), is presented. The beam plasma instability can be excited due to interaction between beam electrons and background electrons and it will produce a new mode. Here, the trapped electrons can be considered as beam electrons with velocity vp (phase velocity of the plasma wave) and density nt (density of the trapped electrons normalized by nc, nc is the critical density for the incident laser light). This new mode will induce a sinusoidal density modulation and then this modulation will produce harmonic wave with modulation wave-number period δk to enhance this new mode. As reported previously, the density modulation can increase Landau damping of the plasma wave and decrease the resonant region where SRBS occurs. Therefore, SRBS will saturate due to beam plasma instability induced by the trapped electrons.