Based on a 54 a(1951—2004)rainfall record of Tianjin with the resolution in minute, the Huff rainfall pattern method was used to analyze the design rainfall pattern and its variations over time. The results show that the most of the rainfall peaks appear in the first and second quarter of the rainfall duration. By comparing the rainfall patterns derived from pre-27 years and post-27 years, it shows the phenomena of the peak rainfalls moving forward and the peak values increasing; in consequence, it was found the phenomena of the peak runoff moving forward, the peak values of runoff being increased and the total volumes of runoff being increased as well. Therefore, even with no change of design rainfall, the rainfall processes and rainfall patterns would have great impacts on the flooding control and the control of annual runoff volume. Therefore, the design rainfall pattern should be updated more frequently as the design rainfall. The hydrological effects of the change of rainfall processes and rainfall patterns should be considered during the design of a construction project.