Here, we report a passively Q-switched Tm laser at 1720 nm. The Q-switching behavior originates from a piece of Tm-doped fiber. Tm-doped fiber has a broad absorption spectrum covering 1.7 μm waveband, which can be used as a fiber-type saturable absorber for 1.7 μm pulsed lasers. In comparison with the typical Q-switching system in which the gain fiber has a different rare-earth doping from the fiber-type saturable absorber, the same rare-earth doping in the gain fiber and the saturable absorber cannot support the effective Q-switching operation. To initiate the pulsing operation in this Tm-Tm laser system, we introduce a mismatch of mode-field area between the gain fiber and the fiber-type saturable absorber. By using ∼50 cm Tm-doped fibers as the saturable absorber, the passive Q-switching 1720 nm laser is realized and produces 340 mW output power with a nanosecond pulse width (400 ns ∼422 ns). After investigating the Q-switching pulses, it is found that the evolution trend of the pulses with the pump power is not consistent with the typical passive Q-switching lasers. In order to understand this unusual Q-switching behavior, we establish a rate equation model that is coupled with the mismatch of mode-field area to give a comprehensive understanding. From the numerical simulation, a new Q-switching laser cavity consisting of dual laser resonance is proposed to guide how to realize a passive Q-switching 1.7 μm laser with a higher output power.