Abstract: For the purpose of evaluating the performance of combined optical diode lasers,two approaches estimating beam propagation factor were analyzed.One was similar to the approach used in the traditional diode stack,calculating the beam width based on the geometry optics,the other was based on the similar Gaussian model of the emitter calculating the beam width by the definition of the second order moment of intensity distribution.Finally the dependence of the M2 factor of the output beam on the number of the diode bars and the number of emitters per bar was deduced.An experiment on a three-bar combined optical diode laser was carried out,with output power of 120W,power density of 209W/cm2,average pitch of 1.1mm,and total beam propagation ratio M2=197.The two approaches were compared through the experiment.The analysis results show that both these approaches are available in estimating the output beam quality of the optical combination diode lasers.