Under normal conditions, the mixing uniformity of boric acid and coolant in the descending section and the lower chamber of PWR is an important factor affecting the core reactivity distribution. In the emergency shutdown process, the transient dilution of boron may cause secondary criticality. Therefore, the mixing of the de-borated slug with the ambient coolant in the reactor pressure vessel was investigated by using a three-loop 1:6 scaled visualization pressure vessel model with ACP1000 as the prototype device. The pressure vessel model retains the structures related to the experiment, such as the radiation monitoring tube, while the other structures are simplified accordingly. The dye solution was used as the boric acid injection coolant to measure the concentration distribution of dyes on the vertical and different cross sections, and the mixing mechanism was analyzed by LIF. In the slug mixing test, the coolant flow rate and the dye injection flow rate were changed respectively, and the chromatid concentration changes with time and its distribution at different times were detected. The effect of Reynolds number is analyzed. In the descending section, the injection continuously collides with the tube wall and is diluted by the surrounding pure water, the transport of the stain along the annulus boundary and the diffusion on the cross section are weakened, and the transport mainly occurs in the circular direction. After the mixed fluid passes through the lower head and the lower plate of the core, the concentration of the fluid region near the wall is lower, while the concentration in the middle is higher, and the distribution is inverted fan. The related research results of this experiment qualitatively reveal the mudding process of boric acid and deborated coolant before entering the reactor core, quantitatively analyzed the concentration distribution of the observed area.