A tunable reflection type band-rejection optical filter based on capillary filled with liquid instead of multimode fiber is projected. The active component of the structure of filtering is a segment of capillary filling liquid and Hg. One end of the capillary is docked co-axial with a single-mode fiber and the other end is sealed with hot melt adhesive. Refractive index of liquid is greater than the capillary refractive index and the capillary core diameter is larger, so there is equivalent to the formation of a large core diameter multimode fiber. On the basis of the multi-mode interference principle, because the capillary liquid core is sensitive to the temperature, the tunable optical filtering is achieved. Simulation and experiment confirm the feasibility of the theory. The filter has some advantages, such as simple structure, low cost and so on, for which insertion loss is 5.12 dB, 3 dB bandwidth is 3.3 nm and the filtering range is up to 7 nm.