Abstract: The three-wave nonlinear effect of radio wave propagation in plasma is one possible solution to the problem of black out. Base on three-wave nonlinear theory, the method for calculating the conversion efficiency is introduced and the nonlinear effect under different plasma electron density model are analyzed. The results of numerical simulation show that there is an optimal signal incident angle, at which the conversion efficiency is maximum; when the thickness of plasma is reduced, the optimal signal incident angle is increased; the larger the derivation value in resonance point of plasma electron density, the greater the nonlinear effect, while the other parameters are identical.