We investigate the behaviour of backward stimulated Raman scattering (BSRS) in inhomogeneous plasma with a transverse external magnetic field via 1D and 2D particle-in-cell simulations and theoretical analysis. The external magnetic field causes nonlinear damping of large-amplitude electron plasma waves (EPWs), and significantly limits the reflectivity of stimulated Raman scattering (SRS) even if is very weak. For a single resonance point, limits the kinetic nonlinear frequency shift, impedes the autoresonance behaviour of SRS, and narrows the resonance region. We also exhibit distorts the distribution function of electrons evidently, and alters the wavefronts of EPWs in high dimensional cases. The simulation results indicate a moderate can significantly reduce the trapped electron induced gain of SRS reflectivity, and make the BSRS reflectivity stay at its linear convective saturation level steadily.