The low Terahertz(THz) radiation power has become the main bottleneck restricting the development of Terahertz science and technology. In order to increase the power of THz source, a novel method of detached cavity is proposed to multiply frequency based on double drift region avalanche multiplier diodes, and then the power combiner of waveguide is designed and realized. The phase of the intermediate frequency of the excitation source is adjusted on the input of the frequency multiplier. And then, the phase of the frequency multiplier output radio source can be controlled precisely. Sequentially, the independent signals of THz which are in-phase can be added together. The efficiency of the power combiner is enhanced avoiding using the phase shifter of THz frequency band. The complexity of the processing is reduced and it is easy to be realized. The simulation results and the measured data show that, the efficiency of the power combining working at THz band is 79%-89% approximately.