Abstract: The fluid flow pattern and free surface fluctuation in a mold profoundly influence both molten steel solidification and molten slag thermodynamics behavior in the continuous casting process. In addition, the fluid flow and surface fluctuation behaviors are important factors of inclusions floatation, meniscus hook formation, and shear layer instability that affect the slab quality. In this work, a 0.6-scale water model of slab mold was built according to a 220 mm×1800 mm actual mold from a Chinese steel plant to investigate the fluid field and free surface fluctuation during the continuous casting process. The characters of fluid flow were studied with particle image velocimetry (PIV) in different submerged entry nozzle (SEN) conditions, including SEN outlet angle, SEN bottom structure, and SEN submerged depth. Simultaneously, the fluctuation behaviors of the mold free surface were monitored using wave-amplitude sensors and analyzed using the F value. The findings show that there are shear flow with tilt down velocity vectors appearing near the one-fourth wide face position of the mold free surface, and some irregular vortexes are detected near the SEN position. In addition, the results show that increasing the SEN outlet angle and submerged depth can expend the up-recirculation flow domain and intensify its buffer action; therefore, the velocity of mold liquid surface decreases. However, changing the SEN bottom structure has negligible effect on fluid flow velocity at the position of one-fourth wide face of the mold free surface, although it can reduce the turbulent kinetic energy of the steel jet. Besides, the variation of surface fluctuate amplitude corresponds to the F value well, and the optimal F, which helps to reduce and avoid the possibility of mold slag entrapment can be obtained in the conditions of SEN outlet angles of 15° and 20° and submerged depths of 135 and 145 mm, respectively.