A method for identifying a host operating system by a Universal Serial Bus (USB) device is disclosed, which pertains to the field of information security. The method includes: A) the USB device is powered on and initiated; B) it is determined whether the first setup packet is a first preset command, if yes, C is performed, if not, D is performed; C) it is determined whether the seventh byte of the first preset command is a second preset value, if yes, an operating system identification is set as a first preset identification, and F is performed, otherwise, E is performed; D) it is determined whether the first setup packet is a second preset command, if yes, the operating system identification is set as a second preset identification, and F is performed, otherwise, E is performed; E) information interaction with the host is performed according to a default operating system identification until the last setup packet is received, and it is determined whether the last setup packet is a third preset command, if yes, the operating system identification is set as a third preset identification, if not, the operating system identification is set as a fourth preset identification; F) information interaction with the host is performed according to the present operating system identification, and then it waits to receive the instructions issued by the host.