A system for purifying rainfall runoff including a bar screen; a collection and diversion device; a filter channel; a grass ditch; and a water storage tank. The bar screen is disposed around the collection and diversion device for preliminarily filtering the road rainfall runoff. The collection and diversion device collects the road rainfall runoff and diverts early rainfall runoff to the filter channel, after the filter channel is full, later rainfall runoff is directly diverted to the grass ditch through open channels. The grass ditch includes a percolation bed capable of filtering the early and later rainfall runoff. The water storage tank is connected to the grass ditch via a guiding pipe, to receive the rainfall runoff effusing out of the grass ditch. Part of the rainfall runoff received by the water storage is transported to the percolation bed by a return conduit.