The process of laser cutting 20 mm thick Q345 carbon steel with high power fiber laser was used to investigate and observe the microstructure of the recast layer and heat affected zone after cutting. The effects of laser power, cutting speed, auxiliary gas pressure, and defocus amount on cutting quality were compared using multiple sets of single-factor experiments. The optimal process parameters in this study were laser power 8 kW, cutting speed 0.6 m/min, Defocus amount +10 mm, oxygen assist pressure 140 kPa, nozzle diameter 1.4 mm, cutting height 0.5 mm. The morphology of the cross-section of the cut carbon steel samples was observed by SEM. The differences in microstructure and size between the recast layer and the heat affected zone and the substrate were found. The formation mechanism of the microstructure in the recast layer and the heat affected zone was analyzed. This research is helpful for the promotion of the application of ultra-high power fiber laser cutting technology in the field of cutting medium and thick steel plates.