The features of Hydrochemical precursory field before the Gonghe M_S7.0 earthquake on April 20, 1990 are studied.The results show that proportion of hydrochemistry observation sites in which trend anomalies appear to all sites in focal region of the earthquake is higher than that far away from the focal region and the proportion reduces gradually as the epicentral distance increases. In the focal area the trend anomalies in which radon content in groundwater goes up gradually emerge earlier than far away from foci. The formation of hydrochemical precursory field has something to do with regional stress field, but has closer relation with focal stress field. Focal stress field may play a main role in the precursory process.