A nonlinear mechanism of the transformation of magnetoacoustic waves to kinetic Alfven waves in the magnetosphere plasma (β < 1) is proposed. A nonlinear dispersion equation describing the decay of a magnetoacoustic wave and the excitement of kinetic Alfven waves is derived on the basis of two-fluid magnetohydrodynamics. We also determined the instability growth rate, the time of parametric instability development and the threshold of the pump wave amplitude.