The present disclosure discloses a method for assuring quality of service in a storage system, where a control node calculates, based on a quantity of remaining I/O requests of a target storage node in a unit time, a quantity of I/O requests required by a storage resource to reach a lower assurance limit in the unit time, and a quantity of I/O requests need to be processed by the target storage node for the storage resource in the unit time, a lower limit quantity of I/O requests that can be processed by the target storage node for the storage resource in the unit time; allocates, based on the lower limit quantity of I/O requests, a lower limit quantity of tokens of the storage resource on the target storage node in the unit time to the storage resource; and sends the lower limit quantity of tokens to the target storage node.