Abstract: The thickness and uniformity of the solidified shell in the mold are essential prerequisites for defect-free billet production. While the influence of submerged entry nozzle structure on flow behavior of molten steel is a key factor causing fluctuations in the solidifying shell. In response to surface longitudinal cracks and breakout incidents observed during the continuous casting of 165 mm × 365 mm rectangular billets, the effects of mold flux lubrication performance, solidified shell morphology, and equipment operating conditions on crack formation were systematically analyzed. The results indicate that surface longitudinal cracks originate from remelting and thinning of the inner-arc shell. Under conditions of inadequate mold flux lubrication and oscillation frame misalignment, these defects are readily initiated and may develop into breakout accidents under severe conditions. On this basis, a finite element numerical simulation was conducted to investigate the effects of straight-through nozzle and bilateral-hole nozzle on the molten steel flow, heat transfer, and solidification behavior in the mold of a small-section rectangular billet. The results show that, for the straight-through nozzle, a concentrated high-temperature core region forms beneath the nozzle, which is unfavorable for superheat dissipation and uniform heat transfer. The impingement of high-temperature molten steel leads to remelting of the inner-arc shell, resulting in a minimum shell thickness of 8.79 mm at the mold exit and an impact depth reaching 0.82 m. Meanwhile, the overall temperature in the upper region of the mold remains relatively low, which is detrimental to mold flux lubrication and inclusion flotation. When the bilateral-hole nozzle is adopted, the high-temperature region within the mold shifts upward and the molten steel impact depth is significantly reduced, leading to a marked improvement in the uniformity of the inner-arc shell thickness. However, the side-port jets may cause erosion and thinning of the narrow-face solidified shell. When the port angle of the bilateral-hole nozzle is set at 25°, the minimum shell thickness at the mold exit exceeds 10 mm, with an average shell thickness of 15.17 mm. Under this condition, the meniscus fluctuation height is 2.53 mm and the average meniscus temperature reaches 1 521 ℃, effectively improving the temperature field distribution and promoting uniform shell growth within the mold.