Here we prepare polycrystalline Mn_1−xNi_xWO₄ ceramics with x = 0, 0.02, 0.04, 0.06 for investigating their magnetic, ferroelectric, and multiferroic properties. The Ni²⁺-substitution gradually expands the lattice and modifies the magnetic performance with increasing x, resulting in a strong modulation of electric properties of those samples. For Mn_0.96Ni_0.04WO₄, ferroelectric polarization is observed in the whole temperature regime below T_AF2, and is well modified by an external magnetic field, which is discussed under the framework of a spin-current model in this manuscript.