To transmit high power optical energy through optical fiber array and aiming at fundamental-mode Gaussian light, a general parameter design method to satisfy the Dammann grating constraint conditions of beam splitting coupling is proposed. First, the components of the system are determined and their mathematical models are given. Then, 1×6 Dammann grating beam splitting is simulated by MATLAB, obtaining sub-spots with beam waist radius of 20.31 μm and distance of pairwise spacing is 127.7 μm, with a total diffraction efficiency of 84.50%, and an inhomogeneity of 0.23%. According to the optical grating diffraction theory and laser divergence angle, the sub-spots radius is calculated. Comparing the simulation results, for incident fundamental-mode Gaussian beam, the sub-spots radius formula derived from plane wave is not applicable. The radius of the sub-spots is inversely proportional to the radius of the incident light and the number of grating periods has no effect on it. And it is proved by experimental result. In order to satisfy the coupling constraint condition, a general system parameter design method is presented, which adjusts the spot radius, waist radius, collimation ratio, grating unit period length, and focal length of the focusing lens, the spot radius and spacing required by precise coupling are obtained.