The conversion of sodium lignosulfonate (SL) into high-value functional materials remains challenging. Hereon, a bio-based waterborne polyurethane (WPU) was synthesized using castor oil and isophorone diisocyanate as the raw materials. SL was used as a functional filler to prepare transparent or translucent composites of sodium lignosulfonate/waterborne polyurethane with good properties through physical blending method. The influence of varying SL contents on the performance of composite emulsions and films were studied. The results indicated that the composite derived from bio-based materials had an occupancy rate of 89.5 %, and all composite emulsions exhibited outstanding storage stability. Interestingly, the addition of sodium lignosulfonate greatly improved the mechanical properties and heat resistance due to the strong interfacial interaction between WPUs and SL molecular chains. Furthermore, the composite film could achieve complete UV shielding performance when the sodium lignosulfonate content reached 9 wt %. This work had significantly expanded the range of high-value-added applications for lignin and potential prospects for its use in the field of sunscreen coatings. •SL is incorporated as a functional filler into bio-based WPUs.•The incorporation of SL enhances the mechanical properties of bio-based WPUs.•SL as functional filler gives bio-based WPUs excellent UV resistance.•The SL/WPU composites can serve as a reference in the field of sunscreen coatings.