Curcumin is a polyphenolic compound that has been widely investigated for its health benefits. However, the clinical relevance of curcumin is limited due to its low water solubility and inefficient absorption. Therefore, curcumin is often encapsulated in nanocarriers to improve its delivery and function. In this study, composite nanoparticles composed of stearic acid-modified chitosan (SA-CS) and sodium caseinate (NaCas) were formed using sodium periodate-oxidized dextran with different molecular weights as a crosslinking agent. The effects of oxidized dextran (Odex) with different molecular weights on the composite nanoparticles were compared. The optimal SA-CS/NaCas/Odex composite nanoparticle (NPO) was obtained using an Odex (150 kDa)-to-SA-CS mass ratio of 2:1. Its size, polydispersity index (PDI), and zeta potential (ZP) were 130.2 nm, 0.149, and 25.4 mV, respectively. The particles were highly stable in simulated gastric fluid (SGF) in vitro, and their size and PDI were 172.3 nm and 0.263, respectively. The encapsulation rate of NPO loaded with curcumin (Cur-NPO) was 93% under optimal ultrasonic conditions. Compared with free curcumin, the sustained release of Cur-NPO significantly reduced to 17.9%, and free-radical-scavenging ability improved to 78.7%. In general, the optimal prepared NPO exhibited good GI stability and has potential applications in the formulation of orally bioactive hydrophobic drugs.