Ultrasensitive and selective determination of biomarkers of the bone metabolism in serum is crucial for early screening, timely treatment, and monitoring of the curative effect of osteoporosis, which is a silent disease with serious health threats. Immunoassay with a simple sensing interface and ultrahigh sensitivity is highly desirable. Herein, a simple electrochemical immunosensor is demonstrated based on gold nanoparticles (AuNPs) electrodeposited on chitosan-reduced graphene oxide (CS-G) composite modified electrode, which can achieve sensitive determination of the important biomarker of bone metabolism, bone gamma-carboxyglutamate protein (BGP). To overcome the agglomeration of graphene and introduce a biocompatible matrix with functional amino groups, CS-G is prepared and modified on the supporting glassy carbon electrode (GCE). Then, AuNPs are electrodeposited on CS-G through their interaction between amine groups of CS. The immobilized AuNPs provide numerous binding sites to immobilize anti-BGP antibodies (Ab(BGP)). The specific recognition between BGP and Ab(BGP) results in a reduction in the mass transfer of the electrochemical probe (Fe(CN)(6) (3-/4-)) in solution, leading to a reduced electrochemical signal. Based on this mechanism, fast and ultrasensitive electrochemical detection of BGP is achieved when the concentration of BGP ranges from 100 ag ml(-1) to 10 mu g mL(-1) with a limit of detection (LOD) of 20 ag ml(-1) (S/N = 3). The determination of BGP in human serum is also realized with high reliability.