The combination of up-conversion materials and photocatalysts can produce the effect of “redshifting” of absorption spectrum, which is of great significance for improving the degradation efficiency of photocatalysts. To improve the utilization of CdS in near-infrared light, the up-conversion luminescent material K3ScF6∶Tm3+,Yb3+ with cryolite structure was synthesized by high-temperature solid-phase reaction method, and a new photocatalytic composite material K3ScF6∶Tm3+,Yb3+/CdS was prepared by combining it with CdS by high-energy ball milling method. The composition, structure and properties were systematically characterized by X-ray powder diffractometer (XRD), field emission scanning electron microscopy (SEM), fluorescence spectroscopy (PL) and UV-Vis diffuse reflection absorption spectroscopy. Simultaneously, the photodegradation efficiency and mechanism of K3ScF6∶Tm3+, Yb3+/CdS to rhodamine B(RhB) were studied. The results show that the optimal mass composition ratio of K3ScF6∶Tm3+,Yb3+ and CdS is 3.6∶1, and the introduction of up-conversion luminescent materials K3ScF6∶Tm3+,Yb3+ synergistically promots the photocatalytic efficiency of CdS. The ·OH and ·O2- participated in the degradation of RhB, and ·OH plays a major role. In addition, under the optimal recombination ratio, the degradation rate of K3ScF6∶Tm3+, Yb3+/CdS samples reaches 99.9%, which is nearly 50 times higher than that of uncompounded CdS after 80 min. All results show that K3ScF6∶Tm3+,Yb3+/CdS photocatalytic composites has potential application value in the field of photocatalysis.