Magnetic-field-induced strain of about 10 percent is reported in Ni48.8Mn29.7Ga21.5 alloy at ambient temperature in a magnetic field order of 1 T. It was confirmed by different experimental methods that the strain is contributed by twin boundary motion. The crystal structure of thermally-induced martensitic phase in this alloy was found to be close to orthorhombic one in temperature range from 245 K to 333 K with lattice parameters a equals 0.619 nm, b equals 0.580 nm, c equals 0.553 nm (relating to the cubic parent phase coordinates) at ambient temperature. More detailed x-ray studies revealed seven-layer shuffling-type modulation along and directions. High magnetic anisotropy properties were found for this phase. The magnetic measurements revealed that the shortest axis (c-axis) is the axis of easiest magnetization, the longest (a-axis) is the axis of hard magnetization, and b-axis is the intermediate one. The orthorhombic phase has low twinning stresses. The compressive stress applied along a-axis of single-variant sample at most 2 MPa is enough to produce approximately 10 percent strain realized by twin boundary motion. The necessary conditions for observation a giant magnetic-field-induced strain in non-stoichiometric Ni2MnGa alloys based on the new experimental data are discussed.