In order to achieve high-power output of fiber lasers with narrow linewidth, the thermal effect and the four-wave mixing (FWM) effect of the Yb³⁺ doped large mode area triple-cladding fiber (LMA-YTF) based fiber laser was investigated. Based on the FWM effect model, the influences on the spectral broadening of the large mode area triple-cladding fiber (LMA-TCF) amplifiers were simulated and analyzed. Thermal distribution model of the LMA-YTF was modeled. The effects of the power ratio in the second cladding on the fiber temperature characteristics and the upper limit of the pump power were analyzed. The influence of the thermal conductivity of polymer coatings and external temperature on fiber temperature was discussed. The upper limits of the pump power of different backward combiners were compared experimentally. The experimental results indicate that the (6+1)×1 triple-cladding backward combiner with a lower power ratio in the second cladding has a higher pump power upper limit than the (9+1)×1 combiner, which agree with the simulation results. A triple-cladding fiber laser is constructed with fully home-made devices, achieving an output power of 6.7 kW and a 3 dB linewidth of 0.32 nm.