The invention discloses an experimental device and an experimental method for measuring a refractive index of a flat transparent medium, wherein a semiconductor laser is arranged on a base; a central line and a cylindrical lens perpendicular to the central line of the semiconductor laser are arranged on the base in a light emitting direction of the semiconductor laser; coordinate paper is arranged on a light incident plane of the flat transparent medium to be detected; and a scale surface of the coordinate paper is stuck to a side face of a light incident direction of the flat transparent medium. The experimental method for measuring the refractive index of the flat transparent medium comprises the steps of sticking the scale surface of the coordinate paper to the light incident surface of the flat transparent medium with a known thickness; lighting laser beam emitted by the semiconductor laser to the cylindrical lens, and lighting an emitted line source to the back of the coordinate paper of the transparent medium perpendicularly, observing a linear light band on the coordinate paper by facing to the incident light on the other side of the transparent medium as a dark band symmetrical to the central line and calculating the refractive index of flat transparent medium according to a formula. The experimental device and the experimental method can be applicable to student optical experiments, classroom demonstrative experiments and engineering field measurement.