We demonstrated a tunable Q-switched Er-doped fiber laser (EDFL) by employing one single mode fiber–step-index multimode fiber–single mode fiber (SMF–SIMF–SMF) structure as both saturable absorber (SA) and filter. Such SMF–SIMF–SMF structure is based on multimode interference. A tunable Q-switched EDFL with a tuning range from 1553 to 1569 nm was achieved by bending SIMF. When the Q-switched fiber laser operated at 1560 nm, a repetition rate of 84 kHz was obtained, together with pulse width and pulse energy of 1.47μs and 104.7 nJ under the maximum pump power of 800 mW. Our results show that the SMF–SIMF–SMF structure is promising difunctional device for wavelength and pulse modulation.