By making use of the parametric-amplif ier model, we analyse the effect of space-charge wave in Raman free-electron lasers. R esults show that without taking the initial transverse velocity into account, kwv20>ωc is a necessary condition under which the scattered wave couples with the slow space-charge wave. The dispersion relation and gain of the scattered wave, as well as the matching condition between the scattered wave and the space-charge wave, are derived.