To investigate the formation mechanism of sink vortex during ladle teeming, the effects of some factors such as Coriolis force, the position of the ladle shroud and initial tangential velocity of the fluid on the vortex formation process have been studied using numerical simulation combined with experiments. In addition, the evolution tendencies of tangential and radial velocities of the fluid over radial position were studied at certain initial tangential velocity. The results show that as for fully settled fluid, Coriolis force is the major reason for sink vortex formation and the spinor near the shroud is the initial driving force. There is no obvious effect of the ladle shroud position on the critical height of vortex for fully settled fluid, while the critical height of vortex significantly decreases with increasing shroud eccentricity for the fluid with a certain initial velocity, and the tangential motion is the main driving force for vortex formation in this case. The initial tangential velocity affects the critical height significantly. The larger the initial angular velocity is, the earlier vortex occurs and the bigger the critical height of vortex is. As a result, keeping the fluid settled for some time is an effective measure to delay vortex during ladle teeming. The relationship between the start height of vortex (H_SS) and initial angular velocity (ω) can be expressed as H_SS=0.11+2.85ω-4.04ω²+1.95ω³, and that of the height of air column extending to shroud (H_CS) and ω expressed as H_CS=0.09+1.49ω-0.79ω², both of the fitting degrees are higher than 0.99.