The causes of nozzles (SENs) clogging during the continuous casting process of aluminium-killed steel are investigated. The equilibrium reactions for Fe-Al-Ca-O-S series are calculated according to the thermodynamic equilibrium model. In order to prevent nozzles clogging, the appropriate scope of w(T(O))), w((Al)s )，w(Ca)/w(Al) and w(S) are proposed. The result is applied in production and good effect is obtained.