Embodiments of the present invention provide a method for writing data into a storage system and a storage system. The storage system includes a redundant array of independent disks (RAID). The RAID includes a control device and X storage devices, a data stripe stripe is stored in the RAID, and the stripe includes T members. The control device determines that at least one first storage device of the X storage devices is out of service, where at least one member is stored in the first storage device; selects a second storage device from the X storage devices, where a quantity of members stored in the second storage device is less than T/X; and then writes target data into the second storage device, where the target data is a data unit or a check unit used to update the at least one member that is stored in the first storage device. This can ensure data reliability to an extent.