A passively Q-switched (PQS) Tm∶YAP laser in the 2 μm waveband is introduced. A linear cavity structure of the Tm∶YAP laser is chosen in the experiment. A laser diode with an output central wavelength of 792 nm is used as the pump source, and a saturable absorber prepared by a new two-dimensional material of black phosphorus is used as a PQS modulation device for the Tm∶YAP laser. In the continuous wave mode, an output power of 1.0 W is obtained with an output central wavelength of 1994.8 nm from the Tm∶YAP laser under the pump power of 8.8 W, corresponding to a slope efficiency of 17.3%. Under the PQS regime, an average output power of 0.9 W and a minimum pulse width of 1.3 µs at 135.8 kHz are acquired from the Tm∶YAP laser at 1986.7 nm under the pump power of 8.8 W, corresponding to a slope efficiency of 14.2%. In addition, the beam quality factor of Mx2=1.10 and My2=1.06 are measured from the PQS Tm∶YAP laser under an average output power of 0.9 W.