A novel ultra-sensitive enzyme-free sandwich-like electrochemical immunosensor was fabricated based on hierarchical AuPd nanochain networks (NCNs). The architectures were firstly synthesized by a single-step seedless aqueous method with theophylline-7-acetic acid (T7AA) as a new capping agent. The obtained AuPd NCNs were used for the antibody immobilization and signal amplification. The immunosensor for carbohydrate antigen 15-3 had a good linearity within the concentration range from 0.001pgmL−1 to 100ngmL−1, with the low detection limit of 0.35fg mL−1 (S/N=3). It showed good reproducibility, high sensitivity and stability, which provided a promising platform for the clinical immunoassay of carbohydrate antigen 15-3.