The invention discloses a storm sewage treatment process and system for an old area of a city. The treatment process comprises the following steps: judging the current weather, and if the weather is rainfall, executing steps 1-2; and if the weather is not rainfall, executing step 3, wherein the step 1 comprises: detecting the storm sewage amount in an intercepting well, comparing the detected storm sewage amount in the intercepting well with a preset intercepting well high water level value, and discharging the storm sewage in the intercepting well to a storage pond if detecting that the detected storm sewage amount in the intercepting well is greater than or equal to the preset high water level; the step 2 comprises: detecting the storm sewage amount in the storage pond, comparing the detection result with a preset storage pond high water level value, discharging the storm sewage in the storage pond to a suspended filtration device if detecting that the storm sewage amount in the storage pond is greater than or equal to the preset storage pond high water level value, filtering, and then draining; and the step 3 comprises: detecting the storm sewage amount in the intercepting well, comparing with a preset intercepting well low water level value, and returning the storm sewage in the storage pond to the intercepting well if the detection result is smaller than or equal to the preset intercepting well low water level.