Abstract: A 1/4 scale water model of slab molds was built to investigate the fluid flow and its symmetry during casting. The model was established according to a 200 mm×2040 mm actual mold. The velocity in the mold was measured by using particle image velocimetry(PIV). By analyzing the velocity field,the fluid flow behaviors such as flow patterns,level fluctuation,time history of speed on symmetrical points,horizontal speed below the meniscus,vertical speed nearby the narrow face and impingement depth could be studied. The effect of casting speed on the fluid flow behaviors mentioned above was investigated. The results show that the PIV technology can not only measure the velocity field,but also show the flow symmetry in a distinctive perspective so that proposing measures for the continuous casting process,such as casting speed,the geometric of the submerged entry nozzle(SEN) and submergence depth of SEN. After analysis,a conclusion can be drawn that the casting speed at 0.5 m·min^-1 is better than 0.6 m·min^-1 under the casting condition. The results show that the PIV technology is a useful method to optimize the continuous casting conditions.