Beam combination is an important method to break through single laser power limit and obtain high power laser output. Brightness is taken as the criterion, and theoretical investigation about the combination effect of different methods is carried out. The pre-evaluation method of laser beam combination effect is given. Relationship among three important parameters used to describe beam quality, including brightness, beam quality factor and Strehl ratio, are analyzed. The formula describing the relationship between brightness and Strehl ratio is given, and brightness scaling magnification formulas of coherent beam combination system and spectral beam combination system are obtained. The power and the brightness scaling amplification capability of the two systems are compared.