We present in this paper the production of beat wave using a single L-band travelling-wave tube (TWT) amplifier. The beat wave was first generated in a primary source using power synthesis method, then injected into a TWT amplifier to obtain relatively high power output. This approach is initiated to provide practical beat wave radiant source for high power microwave (HPM) effect research. Experimental results demonstrate that for 1～2GHz carrier frequency, it is quite practical to produce beat frequency ranges from 1MHz to 900MHz at 10kHz repetitive rate using one TWT amplifier. The output parameters of the beat wave are readily adjustable in terms of varying the feeding frequency components' power levels. The production of beat wave using single TWT amplifier takes advantages over some conventional methods since the beat wave can be formed in low power, low cost microwave sources. This method has been successfully used in certain electrical system HPM vulnerability effect experiments.