A method and a system for controlling quality of service of a storage system, and a storage system. The method includes: collecting information about processing capabilities of the hard disks in the storage system and obtaining processing capabilities of the hard disks according to the information about processing capabilities; dividing a cache into multiple cache tiers according to the processing capabilities of the hard disks; and writing, for a cache tier in which dirty data reaches a preset threshold, data in the cache tier into at least one hard disk corresponding to the cache tier. The method avoids a phenomenon of preempting page resources in the cache.