We have proposed an optically controlled THz power tuning method based on THz wave interference. Through interfered THz waves at the T-junction transmission line, adjustment of the THz power is achieved, and tuning control is attained by varying the optical time delay of the beat-note lightwaves. To demonstrate and verify THz power tuning with an optical delay line, we have also fabricated a monolithic chip on an InP substrate, which integrates arrayed uni-traveling-carrier photodiodes and a planar slot antenna array for 300GHz wave generation. The experimental results reveal that the power transmission of this THz power tuner can be continuously adjusted with a wider tuning range than 7 dB at 300 GHz band. It should be a promising solution for the integration of compact THz wave planar photonic circuits.