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Based on the assumed distribution of plasma flow velocity and temperature, as well as the diameter of wire corona, the magnetic-field evolution during the ablation phase of the wire-array Z -pinch on the 3-MA Angara-5-1 is numerically investigated by means of the rocket model of wire ablation developed by S. V. Lebedev and the 1-D magnetic-field equation. The magnetic-field (current-distribution) evolution process was reproduced, and its changing tendency with the plasma flow velocity and temperature and the corona diameter were revealed. The numerical results show that there generally exist higher magnetic fields and current densities in the precursor column, which are going to be enhanced with the increase of electrical conductivity and plasma flow velocity while to be weakened with the increase of the diameter of the corona and the width of the thermalization layer of the ablated plasma stream, as well as with the decrease of coronal plasma temperature. The plasma in the column consequently tends to exhibit dynamic behavior, such as the experimentally measured contraction, emission, and expansion of precursor column. The current in the precursor column is about a few tens of kiloamperes, and about 5%-10% of the total current takes the path in the precursor plasma at the end of the ablation for the studied current. They would decrease with the increasing diameter of the corona and the decreasing coronal plasma temperature. In the corona region, the magnetic diffusion is dominant, while in the precursor region, particularly in the column, the magnetic convection is dominant. It is found that the corona plasma temperature obviously impacts the current density distribution in the corona region and the precursor one does not.