In the design of dust-bearing sewage treatment plants for coal and ore terminals，the treatment duration of design runoff rainfall determines the treatment scale of sewage treatment plants，but the selection of this value is not specified in the current code. Given the statistics of rainfall in 15 typical port areas，this paper analyzes and compares the rainfall frequency and rainfall. The results show that the average occurrence ratio of rainfall over 2 mm for two consecutive days in each area is large，especially in the south where the ratio is over 50%. The average occurrence ratio of the sum of daily rainfall over 2 mm for two consecutive days exceeding the minimum value of annual maximum daily rainfall is less than 20%，and that exceeding biennial daily rainfall is less than 4%. To ensure that the sewage treatment plant can treat dust-bearing sewage in time and prevent sewage overflow，we put forward the following suggestions：1）The minimum value of the annual maximum daily rainfall，i.e.，H，should take the rainfall corresponding to the design return period not less than two years in the calculation of runoff rainfall. 2）The design treatment duration of sewage plants should not exceed 48 hours.