To improve the coupling efficiency for a Large-mode-area Fiber (LMAF) and a Single-mode Fiber(SMF), a Thermally Expanded Core (TEC) technique was used to expand the core of SMF to implement the mode field adaptation for the LMA double clad fiber 15/130 μm and the SMF 6/125 μm.The transmission loss of the Mode Field Adaptor(MFA) made by TEC processing was analyzed theoretically, and a homemade MFA and a commercial MFA was respectively measured through a self-built fiber laser source with a 1 064 nm central wavelength. Experiment results show that the TEC technique dramatically improves the coupling efficiency of the MFA between the LMA and the SMF. In contract with the commercial MFA, the homemade MFA is capable of a higher operating power, better heat treatment capacity and better reliability. The homemade MFA offers the operating power up to 100 W, and accumulated working time up to 120 min in an air-cooled condition. Furthermore, its single-mode output to be 73 W, and heat treatment capacity to be 27 W. The improvement of the performance of MFA meets the needs of the development of high-power fiber components and are suitable for achieving all-fiber-integration systems of fiber lasers.