<p>Mulberry leaf protein and whey protein isolate were combined to construct a hybrid plant–animal protein nanoparticle system using an ultrasound-assisted pH-shifting technique for the encapsulation of astaxanthin (ASTA). The resulting composite nanoparticles exhibited outstanding encapsulation performance, achieving an ASTA encapsulation efficiency of 89.8%, significantly higher than that observed in single-protein encapsulation. Post-encapsulation analysis demonstrated an increase in the particle size of the composite nanoparticles, alongside an enhancement in the absolute zeta potential value, indicative of enhanced colloidal stability. The free radical scavenging activity of encapsulated ASTA was recorded at 90.69%, markedly surpassing that of unencapsulated astaxanthin. Furthermore, the composite protein nanoparticles displayed improved physicochemical stability under varying pH, thermal, and storage conditions, which contributed to a notable increase in ASTA bioavailability. In conclusion, the ultrasound-assisted pH-shifting method provides an effective nanoparticle. It not only solves key delivery challenges for astaxanthin but also offers a versatile approach for encapsulating other lipophilic bioactives, with promising applications in the development of functional foods and nutraceuticals.</p> Graphical abstract <p></p>