Potash brine seems to be a direct indicator in search for sylvite deposits.The threshold of potassium in a definite area may be called regional criterion of the potash brine indicator,whereas the threshold of potassium defined by the study on minerogenic mechanism of potassium is named general criterion. This paper discusses general criteria for potassium-forming brine and potas-sium-dissolving brine as well as their applying basis---origin of underground water.