Abstract: By considering the effect of suspended solid particles in the ordinary equations for two-dimension in viscid incompressible mixing layer, the Rayleigh equation and the modified Rayleigh equation are obtained And then, by solving the corresponding eigen-value equations with numerical computational method the relation curves between perturbation frequency and spacial growth rate of the mixing layer for the varying particle loading, ratio of particle velocity to fluid velocity and Stokes number are got. Several important conclusions on the effect of suspended solid particles on unstability of the mixing layer are presented in the end by analyzing all the relation curves.