<p>In the current study, a three-dimensional mathematical model combining the large eddy simulation (LES) turbulent model, volume of fluid (VOF) multiphase model, discrete phase model (DPM), dynamic mesh model, heat-transfer model, and solidification model was established to predict the three-dimensional spatial distribution of inclusions during the slab continuous casting (CC) start process. The simulation was divided into four stages according to the actual CC process. The variation of the casting speed and the cooling boundary condition was defined using User-Defined Functions (UDFs). The number of inclusions entrapped by the solidified shell along the casting direction in different stages was studied. Higher casting speeds and smaller inclusion diameters resulted in inclusions being entrapped at greater distances from the meniscus. A method was proposed to calculate the three-dimensional spatial distribution of inclusions from the first slab to the stable slab based on the actual casting time. The number of inclusions showed a fluctuating decreased trend with the increase of the distance from the first slab. The maximum normalized three-dimensional number density of inclusions was found to be located at the top of the first slab. The peak normalized three-dimensional number density also appeared at the positions of 0.5, 1.3, and 2.6&#xa0;m away from the head of the first slab. Based on the prediction of the spatial distribution of inclusions, it was suggested to cut the first slab at 3&#xa0;m to ensure the cleanliness of the CC slab. In addition, the normalized three-dimensional number density of inclusions in the CC slab reached a lower stable value after 10&#xa0;m away from the head of the first slab.</p>