A method and device for installing an operating system are provided. In an example, in a temporary system, a target RAID for installing the operating system and a unique identifier of the target RAID are determined in response to a user operation of specifying a disk array RAID. Then, a mounting point corresponding to the RAID unique identifier is found according to a correspondence between a unique identifier of a RAID and a mount point; and the target partition for installing the operating system is determined in the found mount point, and an identifier is added for the target partition. In a small system for an operating system to be installed, an installation partition and an installation disc where the installation partition is located may be determined by the identifier of the target partition, and the operating system is installed into the installation partition of the installation disc.