The supermode in multicore fiber was analyzed by using coupled-mode theory, and analytical expression for the mode distribution in multicore fiber was derived. The beam quality of the supermode were numerically calculated. Among all the supermodes, only the in-phase mode has preferable far-field intensity distribution, and mode selection scheme using Talbot cavity was proposed and studied numerically on the isometric and ring distribution multicore fiber lasers. The results show that the in-phase mode can be selected effectively in Talbot cavity using appropriate parameters.