The automobile beam steel is mainly used for such structural components as longitudinal beam, lining beam and cross beam on the chassis of heavy truck, so its comprehensive performances are vital to the driving safety and service life of the whole vehicle. The TiN inclusions with large size seriously harm the processability and usability of BT700L steel, while the amount and morphology of inclusions in steel as well as low-temperature impact and corrosion resistance of steel could be improved by rare earth. In the paper, the effects of rare earth Ce on formation of TiN inclusions in BT700L steel are studied based on the industrial tests and thermodynamic calculations. The study results showed that pure TiN inclusions and composite inclusions of Al-O+TiN were mainly formed in the BT700L steel without Ce, while pure TiN inclusions and composite inclusions of Ce-Al-O(-S-Ca)+TiN were mainly formed in the BT700L steel with Ce. The results of thermodynamic calculations showed that precipitation temperature of TiN inclusions could be decreased with the increase of Ce content in BT700L steel so that the precipitation of TiN inclusions was restrained. Such methods as reducing contents of nitrogen and titanium in steel as well as increasing solidification rate could be taken to decrease the sizes of TiN inclusions.