Theoretical analysis on the far-field diffraction of the coherent beam combination (CBC) is presented. An introduction is given to the definitions of filling factor and evaluation parameter of CBC. Based on the theoretical mode and definition, the influence of filling factor on far-field intensity distribution and the relationship between evaluation parameter and filling factor are theoretically described in detail. It is found that the filling factor influences far-field intensity distribution through space modulation factor. Results also indicate that SR is a constant which has not connection with the filling factor, and CR is of a linear relation with filling factor, and NACL has little connections with filling factor, and RCL varies approximately linearly with the square of filling factor, and PIB decreases when filling factor increases in general. If we make the filling factor larger than root 2/2, more than half of the best values of RCL and PIB may be obtained.