The graphite-like carbon nitride (g-C3N4) nanosheets with oxygen-containing functional groups, characterized by with XRD, FTIR and XPS, were obtained by applying chemical oxidation of bulk g-C3N4. The oxygen-containing functional groups not only can be used as anchoring sites for silver nanoparticles (Ag NPs), but also help the formation of high scatter for Ag NPs in Ag NPS/g-C3N4 nanosheets nanocompposites. The weight percentage of Ag NPS in the prepared nanocomposites can be controlled by adding silver nitrate. The Ag NPS/g-C3N4 nanosheets nanocompposites are developed as a superior sensor for the detection of Co2+ ions, which are based upon Raman intensity response of g-C3N4 nanosheets to Co2+ ions. With comparison, it was found that Raman signal increase as the increasing of the concentration in Co2+. Results from the weight percentage sensitivity investigations show that Ag NPS content of 73% in nanocomposites have very low limits of detection (10-9 mol·L-1 for Co2+) in contrast with other Ag NPS content. The sensors also exhibit greater selectivity for Co2+ ions than the other metal ions (Cd2+, Cu2+ and Zn2+). The mechanism is proposed. The localized electromagnetic field is produced around the aggregated Ag NPs with the complexation between Co2+ and N/NH groups in g-C3N4 occurs, so the increase of SERS signal in the intensity can be observed. It is anticipated that Ag NPS/g-C3N4 nanosheets nanocomposites can be a new class of promising material for fabricating SERS sensors.