We have prepared the $\mathrm{graphene}/{\mathrm{MoS}}_2$ heterostructure by a hydrothermal method, and presented its nonlinear absorption parameters and application as a nonlinear optical modulator in the mid-infrared region. Using the nonlinear optical modulator, stable passively Q-switched operation of an ${\mathrm{Er}}^{3+}$-doped ${\mathrm{ZrF}}_4\text{-}{\mathrm{BaF}}_2\text{-}{\mathrm{LaF}}_3\text{-}{\mathrm{AlF}}_3\text{-}\mathrm{NaF}$ (ZBLAN) fiber laser at $～2.8\mathrm{\mu }\mathrm{m}$ can be obtained. The Q-switched ${\mathrm{Er}}^{3+}$-doped ZBLAN fiber laser can yield per-pulse energy up to 2.2 μJ with the corresponding pulse width and pulse repetition rate of 1.9 μs and 45 kHz, respectively. Our results indicate that the $\mathrm{graphene}/{\mathrm{MoS}}_2$ heterostructure can be a robust optical modulator for pulsed lasers in the mid-infrared spectral range.