The invention discloses an open-type Fabry-Perot resonant cavity based on surface wave photonic crystals. The open-type Fabry-Perot resonant cavity is formed in surface wave photonic crystals constituted by a periodical metal columnar structure on a metal plane by reducing the height of a part of metal columns. The metal columns are equally spaced, with the distance being less than a half of the operating wavelength of the surface wave. The resonant cavity has a resonant frequency within a forbidden band of the surface wave photonic crystals constituted by the higher metal columns around. The metal columns with the reduced height have the same height. The open-type resonant cavity designed in the invention is formed by the forbidden band based on the surface wave photonic crystals, has a higher quality factor than a traditional Fabry-Perot resonant cavity and can improve the flexibility of ambient refractive index detection.