Considering the broadband interference caused by the broadcast signal to the parasitic signal in the Parasitic Transmission System, this paper provides an interference suppression technology based on Amplitude Domain Processing(ADP). With the simplified method of Continuous Polynomial Approximation for expressing mathematically the Probability Density Function(PDF), the nonlinear function derived from this PDF can be applied to nonlinear processing in the amplitude domain of the received signal to suppress interference. This technique reduces the error from inaccuracy of PDF estimation and does not need any training sequence. Simulation results indicate that the Parasitic Transmission System with this technique can effectively suppress broadband interference of the QPSK modulated signal, and that the bit error rate(BER) of the system is about 10^-3, even when the interference-to-signal ratio is 20dB.