Abstract: The ship arriving at Shiqiao Lock is random according to the statistic analysis of the data in 2001. Values of the uneven coefficient are different between upward ships and downward ships. The analytical results should be modified by a difference coefficient when the uneven character analysis is based on all the arriving ships. A time guarantee percentage is introduced to denote the degree of daily-arriving-ship quantity deviating from the average level. The higher the time guarantee percentage is, the greater the uneven coefficient is.