<p>With the increasing quality requirements and demand for rotary steel parts, the lack of research on the centrifugal casting’s flow field in semi-filled molds has limited the development of casting parameterization. The development of centrifugal multiphase flow fields, interphase interface formation, and fluid stability in thick-walled pipe fittings were examined. The fluctuation and stability of the flow field during steel and slag co-pouring are innovatively characterized using finite element analysis and casting experiments. The results show that the movement of the conventional semi-filled horizontal centrifugal flow field can be divided into three stages: filling fluctuation period, stable period, and weak instability period. Producers need to control initial solidification during the stable period to avoid defects and performance changes caused by melt instability. The pressure difference due to gravity causes the flow field to shift, and the center of gravity (CG) of the fluid in the stable period deviates nearly vertically and fluctuates in an elliptical shape, tending towards the central axis with increasing mold speed. Experimental castings exhibited eccentric distribution and phase interface fluctuation, while actual oxygen addition, slag fluctuation, and solidification shrinkage caused the CG offset to be smaller than that of ideal pure fluid. Additionally, the instantaneous filling assumption can be used to characterize the stable stage of the melt flow field, with an error of only 1.2% compared to non-instantaneous filling, while reducing computational cost by 30%.</p>