Ultrafast solid-state laser sources operating in the mid-infrared (mid-IR) region are of significant importance in a variety of applications. However, the realization of ultrafast lasers with narrow pulse width at wavelengths between 1.8 and 2 mu m has remained a big challenge due to the strong water molecule absorption. Here, we report a stable mode-locking (ML) pulsed lasing at 1967 nm and narrow pulse duration of 1.97 ps in a diode-pumped Tm:CaYAlO4 (Tm:CYA) crystal laser oscillator. By employing the SESAMs with different modulation depths as mode-lockers, we show that under the increasing saturation parameters, output pulses could be narrowed in the steady state cavity. Furtherly, the generalized Ginzburg-Landau equation is used for the mode-locked pulse dynamic evolutions. It shows that the pulse- regulation mechanism in solid-state cavity at water absorption band is governed by the balance of the laser gain, the modulation depth, and the saturation parameter. To our knowledge, this is the first demonstration of pulse regulation mechanism in the 2 mu m water absorption band by a commercial laser diode in a Tm:CYA laser. Our results provide an effective method for pulse regulation of ultrashort mid-IR pulses and even few-cycle pulses with controllable optical spectra in the water absorption band.