Magnetic-field-induced martensitic phase transition and the concomitant change of volume are investigated in Ni-Mn-In alloy. A well-defined linear relationship is found between the quantity characterizing magnetic degree of freedom and the thermal expansion on behalf of structural degree of freedom, which demonstrates the magnetostructural coupling. Within the exchange-inversion model, such a linear relationship is theoretically derived and the magnetostructural correlation is elucidated. The lattice-entropy change contributes about one half of the total entropy change, suggesting that the magnetostructural coupling plays an important role in the magnetocaloric effect of Ni-Mn-In alloy.