Abstract: This paper investigates the occurrence of unexpected incidents in the enrollment of graduate students on the basis of the Small Probability Event Principle. It has been found that the occurrence of small probability events is accounted for by the following three causes: repetitive effect, accumulative effect and mass inspection effect, and thus to cope with small probability events more promptly and effectively. It is of vital importance for staff in charge of graduate students' enrollment to be highly sensitive to small probability events, to improve the procedure of enrollment, and to set up series of regulations.