In this paper, we investigated the problem of discovering periodicity of a certain event in a binary data series and a new method basing on cross entropy is proposed. First, a series of rational partition methods for binary data series are introduced, which can divide the data series into different segments (partition). Then, we use cross entropy to calculate the partition periodicity, which could be the good measurements for the feasible of event periodicity. Finally, a periodicity evaluation method is proposed to obtain the feasible periodicity of the given event. The results of calculation example show that the method can be used to explore feasible event periodicity in binary data series.