Based on the data of inflow of a sewage plant and data from rainfall monitoring stations in the Jiaojiang District, in Taizhou City in 2014, we analyzed the daily inflow frequency at the sewage plant and the rainwater mixing rate in urban sewage pipe networks, so as to modify the design water quantity of the sewage plant. The results are as follows: the sewage quantities in the cases of light, moderate, heavy, and continuously heavy rainfall were respectively 1. 29, 1. 53, 1. 77, and 2. 31 times those on sunny days. The quantity of groundwater mixed in sewage accounted for 24. 1% of the daily sewage quantity on sunny days, and the quantity of rainwater mixed in sewage accounted for 24. 2% of the daily sewage quantity on rainy days. With the increase of rainfall, the rainwater mixing rate increased from 10% to 35%, and finally tended to be 27%. The rainwater mixing rate of the design water quantity for sewage plants in coastal cities can be determined as a value ranging from 25% to 30%.