Taking the main control sections of the Jing ,Luo and Wei Rivers within Shanxi as examples,this paper has analysed the characteristics of the probability distribution of the low flows in 14 periods; selected the types of the theoretic distribution lines; revealed the change laws of the average values of the period mean flow of various low flow series; the change laws of the variation coefficient ,and the change laws of the deviation coefficient; established the empirical formulae for deriving the low flows with different frequencies and in different periods;and drawn the corresponding relation curves. All the results are very convenient for application.