A novel designing method for optical interleaver based on cascaded solid thin film Fabry-Perot filter is presented. It′s easy to control the thickness of the spacer of the device when employing the fused quartz as the material of the spacer,so the thin film interleaver can satisfiedly meet the demand of the narrow wavelength gap for the DWDM system. The analysis on how to ascertain the thickness of the spacer,the numbers of thin film Fabry-Perot filters and the match of the reflective index of each Fabr-Perot filter's mirrors are given. The experimental result is coincident with the designed one well, and the device achieves to 100 GHz spaced channels, at wavelength around C band, with adjacent channel's cross-stalk less than -24 dB and the minimum insertion loss nearly 0.4 dB or so.