Herein, a facile approach for shape-controlled gold nanoparticle (AuNP) decorated thionine-MoS2 nanocomposites (AuNP-Thi-MoS2) had been reported by using the synergistic effect of MoS2 and Thi. Thi was not only used as an electrochemical indicator, but also as a reducing agent to tune the resulting morphologies of AuNPs. With the ratio of MoS2/Thi decreasing, the shapes of AuNPs changed from spherical, triangle, clover-like to flower-like nanostructures. TEM, XRD and XPS were employed to characterize the formation and chemical composition of AuNP-Thi-MoS2 nanocomposites. On the basis of synthesis, a MoS2-based label-free electrochemical immunosensor had been designed to detect carcino-embryonic antigen (CEA). The proposed immunosensor could detect as low as 0.52 pg mL(-1) CEA with excellent selectivity. Moreover, the expected immunosensor showed high stability and excellent reproducibility, which could detect CEA in human serum with satisfactory results. Therefore, the AuNP-Thi-MoS2 nanocomposites may be considered as a candidate sensing platform for fabrication of simple, label-free and ultrasensitive electrochemical sensors.