A facile and feasible sandwich-type electrochemical immunosensor for ultrasensitive determination of Carbohydrate antigen 19-9 (CA19-9) was designed by using Au nanoparticles functionalized porous graphene (Au-PGO) as sensing platform and Au@Pd core/shell bimetallic functionalized graphene nanocomposites (Au@Pd-Gra) as signal enhancers. Herein, Au@Pd-Gra with a large surface area was prepared for immobilizing plentiful of redox probe-thionine (Thi), horseradish peroxidase (HRP) and secondary antibodies (Ab2), leading to the formation of Au@Pd-Gra/Thi-Ab2/HRP bioconjugate which exhibited satisfying electrochemical redox activity, high electrocatalytic activity and friendly biocompatibility. With the synergistic effect between Au@Pd-Gra and HRP, almost triple amplified detection signal was achieved in the presence of H2O2, so as to improve the detection limit of the proposed immunosensor effectively. Furthermore, Au-PGO was utilized as the biosensor platform which could greatly enhance the surface area to immobilize a large amount of captured primary antibodies (Ab1) leading a further enhancement in the sensitivity of immunosensor. Under optimal conditions, the electrochemical immunosensor exhibited desirable performance for determination of CA19-9 with a wide linearity in the range from 0.015 to 150UmL(-1) and a relatively low detection limit of 0.006UmL(-1). Importantly, the resulted immunosensor displayed good specificity and high sensitivity, implying potential applications in clinical research.