A method of producing beat waves by a single klystron amplifier is introduced in this paper. The experimental results have been obtained that the main frequency of the beat wave is in S-band, the beat frequency ranges from 10 to 120MHz, the peak power of the pulsed beat wave is 1MW, and the maximal repetitive frequency is 500pps. This method has been successfully used in certain electrical system high-power microwave vulnerability effect experiments.