Provided are a method and terminal device for installing an operating system. The method includes: copying an installation boot file in a mirror file of a to-be-installed-operating system into a root directory of a removable storage device attached to the terminal device, the mirror file being stored in the removable storage device; creating an installation script file where a pre-installation script indicates the mirror file is to be mounted to a first folder of the terminal device; designating in the installation script file that a storage address of software installation packages is that of the first folder; adding the installation script file to the mirror file and copying the updated mirror file into a hard disk, where the operating system is to be installed, in the terminal device to obtain a startup disk; and installing the operating system in the hard disk with the startup disk and installation boot file.