This study explores the wavelength-dependent and pulse-width-dependent nonlinear optical properties of liquid-phase exfoliated molybdenum sulfide selenide (MoSSe) nanosheets. The saturable absorption response of MoSSe nanosheets in the visible region is better than that in the near-infrared region, and the response under 6-ns pulse excitation is better than that of a 380-fs pulse. Furthermore, based on the first-principles calculations, we designed a phase modulator and optimized its structure by integrating a monolayer MoSSe into a silicon slot waveguide. The simulation results revealed that the phase shift could achieve a high optical extinction. Consequently, MoSSe exhibits satisfactory nonlinear optical properties and an excellent potential for applications in optoelectronic devices.