Liquid crystal tunable filter (LCTF) is a powerful dispersive element in multi-spectral imaging. Its physical size, however, exceeds the flange focal distance of general commercial off-the-shelf photographic lenses, which makes it difficult to combine with commercial lenses and cameras directly for multi-spectral imaging applications. A relay lens design working at unit magnification is proposed to accommodate the interface. The spectral range is from 400 nm to 720 nm, and the working F number is 4.5. With the symmetrical design strategy, coma, distortion and lateral color are canceled out automatically. The result satisfies the requirements of multi-spectral imaging. This system is compact and low-cost. It is applicable to the design of a portable multi-spectral imager based on the liquid crystal tunable filter.