To the best of our knowledge, the output performance of a self-Q-switched Tm:YAP laser has been controlled by adjusting the cavity length for the first time. By using a concise concave-flat cavity, a pulsed laser emitting at 1993 nm is produced without any additional modulation device. Under a stable self-Q-switched mode, the maximum average output power of 9.76 W is achieved from the laser when the incident pump power is 28.78 W, corresponding to a slope efficiency of 36.9% and an optical-to-optical conversion efficacy of 33.9%. Also, the narrowest pulse width of 485 ns at 48.97 kHz is obtained from the laser with a single pulse energy of 199.3 µJ. As far as we know, this laser has the highest average power and narrowest pulse width compared to other self-Q-switched Tm:YAP lasers.