Abstract: On the basis of water radon data of 33 wells distributed in North China and taking per half of a year as a scale, the evolution processes of the water radon before the Tangshan M7.8 earthquake are studied. The results are: the water radon anomalies were progressively developed from the outer ring to the epicentral area before the Tang shan M7.8, the earthquake occurred in its short-imminent stage, at this stage, water radon anomalies were accumulated at Tangshan area and the anomalous intensity was the bigest.