► Luminescent fibers—Luminescent silk fibers were synthesized. ► Antibacterial—The prepared Ag nanocluster composite silk fibers possessed antibacterial ability. ► In situ growth—It is a simple, low-cost and less time-consuming method for preparing fluorescent silk fibers. The combination of metal nanoclusters with bulk materials is an effective way to achieve better properties of Ag nanoclusters and widening their applications. The common-used Bottom-Up methods for achieving the combination of nanoparticles with bulk materials is inapplicable to the Ag nanoclusters involved systems because the excessive surface modification of Ag nanoclusters tends to form nano-sized aggregates which will lose their luminescence. To solve this, we developed a method for in situ synthesis of Ag nanoclusters on the silk fibers by the ultraviolet light-induced reduction. It not only realized the in situ synthesis of Ag nanoclusters but also finished the combination of Ag nanoclusters and silk fibers. This Ag nanocluster composite silk fibers owned luminescence with the emission band at about 550nm. In addition, we found this novel Ag nanocluster composite silk fibers had strong antimicrobial ability against Escherichia coli and Staphyloccocus aureus. This method is also applicable for most natural and artificial fibers.