Base on the statistical results of increase or decrease of geomagnetic vertical component daily variation amplitude during abnormal geomagnetic daily ratio period,we found that the abnormal high value is caused by the decrease of the geomagnetic vertical component daily variation amplitude during the abnormal day and the increase during the day before anomaly. The decrease of daily variation amplitude of abnormal day may be the main cause of geomagnetic daily ratio anomaly. However,the increase of daily variation amplitude of the day before anomaly is also an important factor too. We established an electromagnetic model to explain the mechanism in this paper,and our results confirm the speculation on the mechanism of this method by previous studies.