The effect of hydrostatic pressure on the martensitic transformation and magnetocaloric properties of MnNi_0.88GeV_0.12 alloy is studied in detail. X-ray diffraction at room temperature shows that the alloy has a single hexagonal Ni₂In-type structure. The phase transition temperature moves to the low-temperature region with the increase in hydrostatic pressure. Under the external magnetic field of 15 kOe, the hydrostatic pressure driving rates $\mathrm{d}{T}_{\mathrm{M}}/\mathrm{d}P$ and $\mathrm{d}{T}_{\mathrm{A}}/\mathrm{d}P$ are 60.25 and 65.25 K GPa⁻¹, respectively, which indicate that the applied pressure expands the working temperature range of magnetic refrigeration. A magnetic entropy change ΔS of −4.8 J kg⁻¹ K⁻¹ is obtained in the external magnetic field of 15 kOe under ambient pressure. The peak value of |ΔS| is increased to 5.6 J kg⁻¹ K⁻¹ under hydrostatic pressure, which is closely related to the structural change of the alloy.