Surface-enhanced Raman scattering (SERS) substrates with high SERS activity and stability are important for SERS sensors. A facile method was developed to fabricate efficient and stable SERS substrates by combining Ag nanoparticles (NPs) and micro-scale sheeted graphitic carbon nitride (g-C3N4). The g-C3N4/Ag NPs hybrid could provide a great number of hot spots and concentrated the analyte by the π–π stacking interaction between analyte molecules and g-C3N4, making a dramatic Raman enhancement. Moreover, the g-C3N4/Ag NPs hybrid uniformly immobilized Ag NPs on the surface and edges of g-C3N4 sheets by an interaction between Ag NPs and g-C3N4, leading to much improved long-term stability. This could be explained in terms of the electron–donor effect of g-C3N4, which was further confirmed by density functional theory calculations. The inherent Raman enhancing effect of g-C3N4 itself also contributed to the total SERS responses. Due to multiple enhancement contributions, the g-C3N4/Ag NPs hybrid exhibited a strong Raman enhancement effect for with an enhancement factor of 4.6 × 108 (evaluated by using crystal violet as a probe), and possessed wide adaptability from dyes, pesticides to bio-molecules.