In this study, we experimentally investigate high-power laser beam combining based on dichroic mirrors. Three-channel 4 kW nearly single mode fiber lasers with central wavelengths of 1059, 1075, and 1090 nm are combined to a single beam. The combined power reaches 11.34 kW with a combining efficiency of 95%. Compared with the sub beam, the beam quality of the combined laser shows gradual degradation as the output power increases and the surface temperature of the dichroic mirror rises. When the full power is reached, the beam quality degrades to M²x=2.941 and M²y=3.183 in both directions. The reasons that affect the combining efficiency and beam quality are analyzed. The effect of substrate thickness of the dichroic mirror on the degradation of beam quality is experimentally studied. This provides an important reference for further optimizing the combined beam quality.