This paper analyzes the far-field intensity distribution of coherent combined beam based on scalar diffraction theory,and proves that the characteristics of unit laser determine effect of coherent beams combination.In this paper,the influence of initial phase,wavefront and polarization of unit laser on coherent combined beam are discussed respectively based on combination of dual laser.And this paper explains why the high power solid-state laser must be based on master of split narrow-linewidth single-frequency seed.Then the design of laser based on the parameters,such as optical path difference,intensity ratio,degree of polarization,are proposed and the ranges of those parameters are presented.