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BACKGROUND Walnut proteins have poor solubility and dispersity under acidic pH conditions, which limits their application in acidic beverages and foods. The present study aimed to fabricate stable nanocomplexes between phosphorylated walnut protein (PWPI) and chitosan (CS) at acidic pH and investigate the encapsulation capacity of the complexes. RESULTS PWPI/CS nanocomplexes prepared at a mass ratio of 2:1 showed small Z-average sizes (approximately 285 nm at pH 5.5 and 222 nm at pH 3.5) with a narrow particle distribution (polydispersity index < 0.3). Caffeic acid phenethyl ester (CAPE) can be effectively encapsulated into PWPI/CS with improved solubility. Circular dichroism analysis indicated that PWPI/CS and CAPE-loaded PWPI/CS (PWPI/CS-CAPE) had a reduced α-helical content and increased β-sheet content. Fourier transform infrared spectroscopy analysis further identified the different driving forces for the complexation of PWPI and CS at pH 3.5 and 5.5 and confirmed the successful encapsulation of CAPE. Rheological results revealed that the PWPI/CS and PWPI/CS-CAPE formed at pH 3.5 (PWPI/CS-CAPE-3.5) had a higher apparent viscosity and better viscoelasticity than the complexes formed at pH 5.5. PWPI/CS-CAPE-3.5 also showed good stability under heat treatment, salt treatment, and long-term storage. Moreover, PWPI/CS-CAPE complexes showed controlled release of CAPE. CONCLUSION PWPI/CS nanocomplexes prepared at acidic pH were stable and promising carriers for CAPE, which would expand the application of walnut proteins in the food industry. This article is protected by copyright. All rights reserved.