In this paper, the different existence forms of radon in underground water are expounded, the mechanisms in which different forms of radon travel and pass the information of crustal stress and strain are discussed and the differences of their reflecting earthquake efficiency are studied. The results show that escaping radon reflects better the crustal stress and strain and earthquake than dissolved radon. Thus, escaping radon can become a new and sensitive reflecting earthquake component of observation net of hydrochemistry for predicting earthquake.