Abstract: The nonlinear dispersion relation of a linearly polarized laser beam in magnetized plasma was derived using the Lorentz transform, in which the nonlinearity of the relativistic factor was taken into account. In terms of Karpman method, the nonlinear governing equation for the amplitude of the linearly polarized laser field was obtained theoretically. Thus, the modulational instability of this intense laser beam in magnetized plasma was analyzed and the temporal growth rate for the finite amplitude of the perturbed field was found analytically. It is shown that the growth rate of magnetic modulation instability increases significantly in the corona region near the critical surface in laser-produced plasma.