Two mode selection methods are studied for 7-core large mode field fiber. One is the traditional Talbot cavity mode selection, and the other is the micro core diameter mode selection. Taking seven core fiber as an example, the mode selection mode of traditional Talbot cavity is theoretically analyzed and simulated, and the coupling factors between each super-mode and other modes in multi-core fiber is analyzed. In addition, a new mode selection mechanism is analyzed, that is, micro core diameter fiber is used for mode selection. Compared with the traditional Talbot cavity, it is proved that the mode selection mechanism of micro core diameter fiber is more convenient and simple to select the in-phase super-mode in multi-core fiber, which has great potential in practical application.