This work took advantage of 3D biomimetic hydrangea-like bismuth oxychloride (BiOCl) and PtNi nanocubes to fabricate a facile electrochemical immunosensor for the detection of CA19-9. Herein, APTES functionalized BiOCl (APTES-BiOCl) with a large specific surface areas can provide an eco-friendly microenvironment for biomolecules. The PtNi nanocubes prepared by the classic hydrothermal method have good conductivity, which can facilitate electron transfer highly efficient. Meanwhile, PtNi nanocubes can immobilize the anti-CA19-9 via covalent Pt-N bonds to anchor a great number of biomolecules more firmly. In addition, reduced graphene oxide (rGO) with excellent conductivity was implemented as a matrix, which greatly promoted electron transfer at the sensing interface. Under optimal conditions, the proposed immunosensor can detect CA19-9 sensitively, from which a linear range of 0.01–150 U·mL–1 was derived accompanied with a low detection limit of 0.0034 U·mL–1. The results illustrate that the designed immunosensor exhibited excellent specificity, good reproducibility, and satisfactory stability. Moreover, it can be applied in the detection of real samples, which indicates that the immunosensor has great potential for early screening and diagnosis of diseases.