In order to cut off the Q-switched pulse trailing and improve the electro-optical efficiency, an exciting mode of synchronous modulation in a mechanical Q-switched radio frequency (RF) excited waveguide CO2 laser was proposed. The effects of synchronous modulation signal on the tail length of Q-switched pulse, pulse width, peak power and electro-optical efficiency were studied experimentally. The results show that, when the cut-off time of synchronous modulation signal is -80μs, the tail length of modulation Q-switched pulse is the shortest of 23μs. The electro-optical efficiency is the highest of 3% when the non-excitation duration of synchronous modulation signal is 800μs. The excitation mode of synchronous modulation can effectively cut off the pulse tail and keep the pulse width at the same time, and improve the peak power and electro-optical efficiency.