We have successfully demonstrated a stable dual-wavelength Q-switched erbium-doped fiber laser (EDFL) using a single mode fiber-multimode fiber-single mode fiber (SMF-MMF-SMF) structure-based filter. Using a graphene oxide (GO) saturable absorber (SA) to modulate the cavity loss, passive Q-switching of the dual-wavelength laser is achieved at 1 549.6 nm and 1 558.6 nm. The laser recorded the shortest pulse width of about 2.9 μs, the maximum pulse repetition rate of 65.27 kHz and the maximum average output power of 0.99 mW at pump power of 225.1 mW. The present laser has the maximum pulse energy of 15.17 nJ. A 2 SMF-MMF-SMF structure has been experimentally confirmed to be very promising as a wavelength filter.