Abstract: Since the density distribution of the outer flow field of a blunted cone flyer is inhomogeneous, the beamlet deflection in the far-field target surface occurs and the detection accuracy of the optical device mounted on the flyer is affected. A method of ray tracing in inhomogeneous medium was investigated for tracing a slim laser beam in the steady outer flow field of the blunted flyer in high speed movement. The beamlet path in the outer flow field of a blunted flyer was calculated. The result shows that in the chosen location, the effect of the outer flow field of a blunted flyer on the exit angle of the beam is observable, but not dramatic; the variation of flow field density gradient and the beam deflection of the beam exit angle increase when a cooling source is applied on the surface of the blunted flyer. The result provides a reference for correcting the beam deflection of the optical system mounted on blunted flyer caused by outer fluid field.