Under the condition of high power laser operation, the quantum defect of rare earth ions and the intrinsic absorption of glass materials will lead to the overall increase and gradient distribution of the temperature of the gain fiber in the optical fiber amplifier. In the state of thermal equilibrium, the thermo-optic effect of optical fiber materials will induce the redistribution of transverse refractive index of optical fiber and cause the change of mode characteristics of gain fiber under the condition of high power laser operation. For this reason, the numerical calculation method of multi-physical field finite element modeling is used to systematically study the thermally induced mode characteristics of large mode field ytterbium doped quartz fiber under the condition of high power laser operation. The changes of mode characteristics of large mode field gain fiber under different laser operating power, gain fiber design parameters (core diameter, numerical aperture, thermo-optical coefficient) and fiber bending are analyzed and summarized. The results show that with the increase of laser power, the temperature difference between the core and the cladding will increase, which leads to the increase of the parameter V of the fiber, the decrease of the transmission loss coefficient of the mode and the increase of the power factor of the mode in the core region.