Semiconductor photocatalysts with specific facets can induce high reactive activities has aroused wide attention. Here, we endeavor to gain quantitative insights into the intrinsic facet-dependent catalytic activities of K3B6O10Br (KBB) crystal using in situ Raman technique under room temperature by photocatalysis dechlorination of 2,4-DCP as a model reaction. Using a well-defined sizable KBB single crystal (size: 28 mm X 20 mm X 9 mm) with high (211), (110), and (101) facet exposure, the time-resolved Raman spectra for different facets have been clearly tested, it shows that the Raman spectrum of (211) facet had a remarkable change compared with (110) and (101) facets when the crystal was immersed in the 2,4-DCP solution under light irradiation. Through DFT, we obtain qualitative details on the reaction mechanisms of photocatalyzed and provide a refined understanding of the elementary processes. It was found that the -OH contact mode between the pollutant and the crystal facet was the most effective mode, which can produce more (OH)-O-center dot radicals than the other two modes. Moreover, the (211) facet offers the largest ratio of K atoms and surface energy, making the (211) facet more active than (110) and (101) facets.