The Khorat basin, which hosts major potash deposits in Laos, is one of the largest potash ore cluster areas in the world. The main mineral assemblages are rock salt, carnallite and secondary potash. Fissure water is mainly distributed in potassiummagnesium salt layers with a depth of 150 m. Leaking of fissure water has imposed serious threat on production safety. But it is still unclear about origin and evolution process of fissure water. This study carried out hydrochemical and H and O isotopic analysis for 12 samples of waters from orebearing layers and neighboring water bodies. The results show that fissure water is saturated brine with high TDS (368.1-430.7 g/L). The fissure water, similar to salt springs, belongs to chloride- type water. The chemical characteristic coefficients of constant and trace ion contents indicate that the fissure water is affected by dissolution of potassium and magnesium salt minerals. The δD (-64.2‰~-55.2‰ average -60.42‰) and δ18O (-7.75‰~-7.1‰, average -7.34‰) of fissure water are different from that of original brine in rock salt (δD=-144‰~-78‰，δ18O=-1.1‰~4.2‰) and potassiummagnesium salt (δD=-54.75‰~-1.42‰，δ18O=-7.09‰~0.95‰) at the depositional stage, and are plotted close to the global meteoric precipitation line. This suggests that the fissure water is not characterized by original residue brine, but mainly originated from minerals of evaporite, leached by meteoric precipitation. The determination for source of fissure water will provide some basis for understanding groundwater cycle and treatment of leakage of mine water during development process of potash deposits.