Based on statistical test method, sampling error distribution in hydrological frequency analysis has been studied. The test results show that: a) Design value is estimated “systemic” smaller than actual value according to smaller sample size. The sampling relative error (SRE) is below zero. With the sample size increasing, deviation gradually reduce, and SRE is characteristics of zero mean normal distribution. b) SRE would significantly increase with large value of Cv and Cs. So additional revision of SER should be considered in small sample size condition.