Abstract: In the research of frequency selective surface(FSS), miniaturization design has always been one of the hot topics of research, and the miniaturized unit structure can greatly improve the stability of FSS. In this paper, a liquid crystal material is used to design a tunable FSS unit based on a three-layer mutual coupling structure. In order to connect the metal structures of each unit, a lumped element is added to feed. The results show that the size of the structural unit is equivalent to 1/16 of the operating wavelength, and the continuous tunable range is up to 11.3%. The structure designed in this paper has the characteristics of miniaturization and tunability, which provides a new design idea for the design of FSS, and has certain theoretical guiding significance and practical value.