Abstract: In the karst groundwater exploration study of the Mouthspring Cambrian-Ordovician (∈-O) carbonate aquifer system which is situated on the southwest of Datong of Shanxi Province, environmental sulphur isotopes were used for identifying the origin of karst groundwater, recognizing the environment of karst groundwater with three kinds characterized by its circulation, discharge and recharge, distinguishing the sub-systems of karst groundwater with its mutual relationships. The difference of enrichment of 34 S in species with different valences, the fractionation of sulphur isotopes mainly the isotopic variations of sulphur compounds including δ³⁴S(SO₄^2-) and δ³⁴S(HS^-) were used for this purpose. The Mouthspring ∈-O aquifer system is different from two neighbouring ∈-O aquifer systems, the Deityhead and the Threefloor, using the relationships of their sulphur isotopes, δ¹⁸O(SO₄^2-) and ¹⁴C. Large variations of δ³⁴S(SO₄^2-), δ³⁴S(SO₄^2-) and δ¹⁸O(SO₄^2-) were found in karst groundwater, however, the anomalous values were occurred in the Z1 spring of Deithead ∈-O aquifer system.