选取中国大陆有代表性的31次M_S≥5.0地震，分别进行G-R公式的地震拟合优度计算，并对计算结果进行逐一分析，结果发现，77%的地震震中周边一定区域内地震拟合优度在震前表现出明显下降异常，同震或震后1—2个月内再次出现明显下降的，数年内周边500km以内往往有中强地震发生。异常出现的时间、形态和幅度等与地震震级、发震构造等关系不明显。部分地震震前异常不明显，可能与震源深度、监测能力等有关。

By selecting 31 representative earthquakes of Ms≥5.0 in China mainland and calculating seismic goodness of fit based on the G-R relationship, we got the feature of seismic goodness of fit before mid-strong earthquake and then analyzed on the result one by one. The results showed that 77% of seismic goodness of fit was dropped obviously before earthquake at the adjacent areas around epicenter, and there always will be mid-strong earthquake within 500 kilometers around epicenter if seismic goodness of fit was continuously decreased. The relationship between the emergence time morphology amplitude of abnormality and magnitude causative structure was not obvious. some anomalies before earthquake were ill defined, the phenomenon maybe  related to focal depth and monitoring ability.