Abstract: The complex water supply modes in the water diversion system of hydropower plants with multiple units cause interactions among the hydraulic coupling units, which change the dynamic behavior of whole hydro generation system, and have great effects on the control tuning of governors. The relative gain array（RGA） method provides a measure of interactions among control loops in a coupled system. In this paper, RGA method was applied to quantify the control interactions among different units in a hydro-coupled power plant. Based on the state space equation of the whole hydro-coupled generation system, RGA matrix of the coupled system was calculated in frequency domain. Distribution properties of the interactions in frequency domain had been analyzed and influences of the surge tank and the branch penstocks on the degree of such interactions had been proved. Time domain simulations indicate the practicability and effectiveness of the RGA method and these interaction analyses will provide basis for coordinated control design for multi-units in hydro-coupled power plants.