Abstract:The research on earthquake precursors is the foundation of earthquake forecast, but it is composed of a number of careful case studies low. The research on earthquake precursor must be put in advance of the attempt of earthquake forecast. Criterion is needed for the judgment of a possible earthquake precursor. Neither any academic authority nor any prevailing hypothesis can give a terminal conclusion. The cases are not yet as enough for credible statistics. Instead, a natural procedure are adopted to identify the anomalous changes in observation, in which one has to 1) demonstrate a normal background, 2) exclude interfering influences, and 3) show the relevance to the earthquake. The earthquake-relevance plays the key role. Reliable earthquake precursors require accurate observations. The demand of research on earthquake precursor is that a priority in planning, as well as drafting technical standards, future observations aiming at earthquake forecast should be given.