To study the limits of improvements of the high-power large-field fiber power and brightness caused by the thermo-optic effect and thermal-induced mode instability under strong pumping conditions, we analyze the influence of the variation of absorption coefficient on the thermal deposition, thermal-induced refractive index, and numerical aperture of a fiber. Results show that a fiber with high absorption coefficient causes a high heat load density, and its numerical aperture increases under the thermal-optic effect modulation, thereby reducing the mode instability threshold. Based on the theoretical research, fibers with two different pump absorption coefficients are designed and fabricated,and high-power mode instability experiments are performed. Experimental results show that the mode instability threshold power is 800 W for the high pump absorption fiber with an absorption coefficient of 1.71 dB/m. However, for the fiber with a low pump absorption coefficient of 1.20 dB/m, the mode instability is not observed when the output power reaches 1700 W. Thus, the mode instability threshold can be considerably improved by decreasing the pump absorption coefficient of the active fiber. These results are significant to the development of high-power active optical fibers and provide a novel and effective technical approach for obtaining high-power fiber laser output.