A gas-gas two phase composite beeline benchmark method and a device based on external supporting belong to the technical field of precision instruments and measurement. Two assistant gas films are respectively constructed along X direction and Y direction on the external neighboring two sides of a flotation guide cover and opposite to the main working gas film supporting direction, and acts on thetwo sides of an orientation flotation board with a main working gas film; a positive working gas film and negative direction working gas film are formed on the oriention flotation board working surface, the inside main flotation rail and the outside assistant rail working surface, respectively, the working pressure of the two gas films are Pinside and Poutside, respectively, the oriention flotation board works under the composite effect of double direction working gas film; the outside of the flotation guide cover is provided with an assistant slideway A and an assistant slideway B in the device, and the flotation guide cover is also provided with a throttle structure facing to the main floatation slideway, the assistant slideway A and the assistant slideway B. The invention constructs avertical beeline benchmark, which has the advantages of high stiffness, strong anti-jamming capability and good working stability and the like.