A new optical reading system that can read the fluorescent signals of microarrays on glass slides with large detection area and high sensitivity is proposed. This system uses gratings (1200 g/mm) with several micron thick PDMS film on surface. Normally incident laser was coupled into a thin glass slide by grating coupling and confined in the slide due to the total internal reflection (TIR). The TIR generates evanescent planar wave on the glass surface. The evanescent wave was employed to excite the fluorescent signals in the microarrays. The method takes the advantages of low excitation background, large and uniform detection area.