The reconstructingprocess of the branch lines of a continuous wave CO2 laser is observedexperimentally with the help of a FMCW laser radar system. It is considered that thebranch line is stabilized when the heterodyne signal of a far-field target appears. It isconcluded that different branch line takes different time to reach its stability, and thetypical time is about 20 μs. Especially, the end of the competition between branch linesdoesn′t mean thecoherence reaching its best situation. The observation shows that it takes at least 6 μs to reconstruct the coherence ofa branch line.