The present work is aimed to analyze the influence of the mixing between slat wake and main element boundary layer on aerodynamic force of the main element of multi-element airfoils. In order to alter the mixing condition of slat wake and main element boundary layer, slat wake varies with different momentum coefficients jet which applied at the trailing edge of slat. Numerical computation is used for solving two dimensional unsteady flow of multi-element airfoils model 30P30N at different jet momentum coefficients, the numerical results indicate that: the onset location of mixing between slat wake and main element boundary layer move downstream along main element chordwise direction, the suction peak of main element upper surface is improved, and the main element lift is enhanced with the jet momentum coefficients increased; onset location of mixing has effect on suction peak and lift of main element; the mixing of slat wake and main element boundary layer is restrained by angle of attack increasement.