Two-dimensional transition metal dichalcogenides are considered promising materials for next-generation photonics and nano-optical devices. Although many previous reports have shown saturable absorption of molybdenum diselenide (${\mathrm{MoSe}}_2$), these nonlinear optical (NLO) properties of ${\mathrm{MoSe}}_2$ were measured in separate works and under different conditions with their hot-carrier relaxations. Here, we conducted a series of coherent studies on the NLO properties of few-layer ${\mathrm{MoSe}}_2$ via open-aperture $Z$-scan and degenerate pump-probe techniques. These measurements were taken to test the materials’ capabilities as a slow-saturable absorber. A slow-absorber model was employed to analyze the NLO measurements, and the results show that the NLO modulation depth was modeled to be 7.4% and 15.1% for the linear absorption coefficients of $5.22{\mathrm{cm}}^1$ and $6.51{\mathrm{cm}}^1$, respectively. The corresponding saturated intensities were modeled to be $39.37\mathrm{MW}/{\mathrm{cm}}^2$ and $234.75\mathrm{MW}/{\mathrm{cm}}^2$, respectively. The excitation carrier recovery time of few-layer ${\mathrm{MoSe}}_2$ was measured by degenerate pump-probe techniques to be $～220\mathrm{ps}$. These nonlinear optical performances make it a promising slow-saturable absorber for passive mode locking in femtosecond lasers.