Outstanding performance in lithium-ion-battery (LIB) of Ce-doped SnS2 nanostructure is attributed to Ce[Formula: see text]. Based on DFT, the stability, electronic and magnetic properties of Ce-doped defective/pristine SnS2 monolayer are calculated. Comparing different doping configurations with/without S-vacancy, the 2Ce atoms locating near S-vacancy (V[Formula: see text]) in SnS2 monolayer is the most stable configuration. The two localized Ce_4[Formula: see text] electrons contribute to the ferromagnetic state of this compound, and confirms the Ce[Formula: see text] not the common Ce[Formula: see text] oxidation state, i.e., 2Ce[Formula: see text]+[Formula: see text]2SnS[Formula: see text][Formula: see text]S(or[Formula: see text]+[Formula: see text]V[Formula: see text])[Formula: see text]=[Formula: see text]Ce[Formula: see text]S3[Formula: see text]+[Formula: see text]2Sn. Larger ionic radius may offer more lattice space for Li[Formula: see text] and coincide with present experimental report. This work clarified the positive effect of intrinsic defect in SnS2 monolayer and will stimulate further experimental and theoretical investigations in this field.