Abstract: A multi-aperture array coupling directional coupler was designed to measure the output power and monitor the spectrum for the 8 mm-band gyrotron. The formulas of multi-aperture coupling coefficient were derived based on loose-coupling theory. Coupling characteristics of equal-radius, binomial, Chebyshev and Gauss distributions were compared by numerical computation with Matlab and simulation with CST code, respectively. Then the composite distribution was proposed. The simulations demonstrate that the composite distribution obtains the advantages of multiple distributions simultaneously. It is beneficial for the improvement of directivity and has smaller ripples in inband coupling coefficient along with broader effective band. In addition, appropriate distribution of multi-aperture array such as Chebyshev distribution and proper distance between the two holes closest to the edge can improve the directivity effectively.