Electronic Jamming is an important means to cover the penetration formation and improve survival probability of penetration aircraft when Air force performs ground penetrating mission. A compound interference method of jamming surface-to-air missile guidance radar by scattering long –range jamming signals though chaff clouds is proposed，which uses jammer to irradiate chaff clouds scattered on the penetration route, and injects jamming energy into the main lobe of surface-to-air missile guidance radar by scattering chaff clouds to make better interference effect. On basis of this method, a chaff projectile throwing model which can calculate Si effectively is proposed to guarantee the continuous and effective jamming to surface-to-air missile position when penetration formation carry out frontal penetration, It is proved through example that the model is reliable.