Aiming at the sags and longitudinal cracks of 15CrMoG rectangular billet， the melting temperature， alkalinity， viscosity and other technical parameters of protective slag were designed. By adjusting the alkalinity of the protective slag from 0.85 to 1.15， the melting point from 1 132 ℃ to 1 200 ℃， and the viscosity from 0.67 Pa·s to 1.4 Pa·s， the crystallization rate of the protective slag was increased， the thickness of solid-state slag film was increased， the thermal resistance was improved， and the heat transfer was uniformly stabilized. At the same time， the melting rate of the slag was optimized from 70 s to 32 s， vibration parameters were controlled， non-sinusoidal vibration bias slope was increased， slag consumption was increased， and lubrication was improved. The surface sags and longitudinal cracks of the billet were eliminated by optimizing the physicochemical parameters and vibration parameters of the slag. The proportion of defective casting billet finishing treatment was greatly reduced， which reduced the grinding work of casting billet and improved the metal recovery rate.