The physical process of filled-aperture coherent beam combining (CBC) using diffractive optical elements (DOEs) is analyzed. The mathematical model of CBC based on DOEs is build up, and the relationship between the complex amplitude of the combined beam and the phase distribution of the DOEs is deduced. The uniformity of the intensity of combined beam is used as the evaluation function of the iteration, and the phase distributions of the one-dimensional diffractive beam combiners are calculated. Compared with the reported DOEs beam splitters, these beam combiners can achieve higher combining efficiency. The phase distributions of the beam combiners are optimized using both simulated annealing algorithm and stochastic parallel gradient descent algorithm, and the computational efficiency is significantly improved. The phase distribution and combining efficiency of multi-beam diffractive beam combiners are presented. The impacts of single disabled beam and surface error of the DOEs on the combining efficiency are analyzed. The simulation results expose that with the increasing number of component beams the impact of single disabled beam on combining efficiency decreases, and the RMS wavefront error of the DOEs should be less than 1/28 of the wavelength in order to make combining efficiency degradation less than 5%.