The invention provides a dredging system and a dredging method. The dredging system comprises a sewage tank, a water inlet pipeline, a spraying component, a drainage pipeline, a sludge discharging channel, a sludge collecting container and a sludge discharging device; the water inlet pipeline extends into the sewage tank to be connected with the spraying component; the spraying component is arranged on the upper portion of the interior of the sewage tank; the drainage pipeline and the sludge discharging channel are connected with the lower portion of the sewage tank, and the junction of the sludge discharging channel and the sewage tank is lower than the junction of the drainage pipeline and the sewage tank; the drainage pipeline is used for draining sewage generated after scouring of thespraying component out of the sewage tank; the sludge discharging channel is used for discharging sludge settling in the sewage tank into the sludge colleting container; and the sludge discharging device is used for discharging sludge in the sludge collecting container to an oil separator. Dredging without opening the tank is achieved, the operation efficiency is improved, the labor intensity of workers is relieved, the safety risk of manual tank opening for dredging is eliminated, the dredging frequency of the sewage tank can be increased, and the load of a downstream sewage disposal system is reduced.