Mode filtering by gain structure in multicore fiber laser is studied. The supermode distribution of a 7-core fiber laser is calculated by the vector coupled-mode theory, and gain coefficients for different supermodes in different power-levels are calculated. It is shown that in a low power level, gain coefficients for different supermodes almost equal to each other, the far-field laser profile emitted from the multicore fiber laser will give mix-mode character. When the laser output power is in a higher level, the gain coefficient for the in-phase mode is the largest among all the supermodes, which will dominate the mode competition. The far-field laser profile will show in-phase mode character. The validity for the mode filtering by gain structure is discussed.