© 2018, Springer Nature Singapore Pte Ltd. In this paper, we proposed an improved percentile method that used for calculating the threshold value of the extreme precipitation event. The method normalizes the series of daily precipitation data by Box-Cox transformation and calculates the extreme precipitation threshold through the Z-score and specified percentile. According to the experimental result, the proposed method shows higher stability under some circumstances, comparing to the traditional percentile methods. As the complement, it is capable to obtain better results in the calculation of the extreme precipitation threshold if combined with other percentile methods.