Alert degree refers to human in perform the task duration time to maintain the level of attention or vigilance. At present, the EEG is the most common and most objective research alert degree physiological indexes. When the alert degree has change, the EEG signals recorded the pattern change accordingly. Therefore, through the records and analysis in the process of man on a mission of EEG signals can estimate alert degrees. This paper introduces the EEG and alert degree of the basic knowledge, the alert degree analysis based on an EEG with estimated the latest research methods, key technology and possible applications for reviewed in detail, and discusses several now needs to solve the problem.