It has been known that tunable optical filter technology is one of the most critical techniques for wavelength-division multiplex (WDM), and the tunable optical fiber filter technique is extremely important for the development of all-optical networks and fiber-optic sensors. A new tunable band-rejection optical filter based on large core-diameter multimode optical fiber is projected. The filter is realized by a single-multi-single (SMS) mode fiber optical structure, which is composed of a multimode fiber with the cladding/core diameter of 125/105 μm and standard single mode fiber, with the end interface not welding together. Based on the multimode interference principle, the structure implementes the filtering by use of the structure sensitive to the stress. A broadband source which is amplified spontaneous emission (ASE) and an optical spectrum analyzer are used to monitor the transmission spectrum. Experimental and simulation results are given to prove the feasibility of the improved SMS structure.