The refractive index of the coating is one of the essential parameters used for principal strain separation in luminescent photoelastic coating (LPC) method by oblique incidence technique. According to the theoretical analysis of the amplitude of the emitted light exciting the coating, which returns along the same path as the oblique incident excitation light, we propose a new online method of measuring the refractive index of the LPC based on optical Fresnel response of the coating. The refractive index of the coating containing Rhodamine B as luminescent dye, is measured under the excitation at a wavelength of 465 nm and an incident angle of 60. The experimental result proves the feasibility of the new method, and the present method is also applicable to the refractive index measurement of other luminescent coating.