The monthly temporal and spatial characteristics of hail frequency (HF) over Shandong Province from 1999 to 2017 are statistically analyzed based on observation data. In addition, the reasons of the monthly variations are explored by the objective classification of circulation patterns and the composite analysis of physical characters based on atmospheric reanalysis data. The statistic results show that there are obvious monthly variations of HF and spatial differences in the monthly hailfall area over Shandong. The exploration of formation mechanism is shown below. 1) About 88% of hail occurs in the trough or the northwest airflow behind trough, and the monthly variation of the above two circulation patterns' cumulative frequency is consistent with the monthly variation of HF. 2) Low-level water vapor and thermal conditions affect the HF. From April to June and from September to October, sufficient water vapor and strong instability correspond to higher HF. The HF is the highest in June because both water vapor and K index reach their maximum and the positive anomaly is the smallest. 3) From April to September, the spatial distribution of hailfall area is well correlated to low-level water vapor transport and convergence, while such correlation is not obvious in October.