In order to obtain more detection channels and simpler structure, bi-color optical route and bi-color detection can be used in modern remote imagers. But this technology can not come into reality without bi-color thin film filters. To design such a filter is not easy since it needs a much larger number of thin film layers. In this paper the author discusses different design methods and points out that the periodical symmetrical stack is more practical to design such a filter. Numerical design example for 3.5~4.0 micrometer and 8.0~9.0 micrometer channels is given with two often-used infrared coating materials ZnS and Ge. As made in other filter design, different optimization processes were made to obtain more ideal result. Practical fabrication of this filter was made to test the numerical design result with a box-type vacuum coater. Experimental results show that the peak transmittance of both channels exceeds 70%, with a cut-off transmittance less than 1% from visible region to near 11 micrometer which meets approximately the requirements of remote imagers.