This paper, in order to improve the front wave of the horn antenna and to get the higher gain, presents a three-layered metallic grid meta-material structure. Both the theory analysis and the retrieved effective refraction show that the zero index refraction occurs at about 7.1 GHz. A prism experiment is carried out to verify the properties of this zero refraction. When this structure is used as the superstratum of a horn antenna, the front wave phase of the antenna is changed from spherical wave to plane wave, the angle of the beam-width becomes narrower and the gain of the antenna is increased by 2.6 dB. The effect of zero-index metamaterial can improve the front wave of the antenna, which provides a method for realizing the higher gain of horn antennas. This metamaterial will have some applications in practice.