This invention is relative to a predistortion for PWM amplifier and its predistortion method. The predistordion comprises: a PWM modulator model and a gain attenuator. The above PWM model comprises complex numbers of multipliers, complex numbers of FIR filters and a adder. The above complex numbers of multipliers is used to implement the signal with high order power for input signal, the above complex numbers of FIR filters each makes the filtration to the signal with high order power, the above adder takes adding operation to the input signals and the output of the complex numbers filters and outputs prestortion signal. The above gain attenuator compares the above predistortion signal with input signal, calculates the real-time gain, and adjusts the input signal due to the real-time gain, then outputs the adjusted signal to the PWM amplifier.