A mode multi/demultiplexer based on a three-core optical fiber that three cores are two single-mode cores and a few-mode core is proposed. Apply a long-period fiber grating to realize mode conversion between LP01 mode in the single-mode core and LP11 mode in the few-mode core. Launched LP01 mode from two single-mode cores simultaneously, can stimulate two degenerate LP11,a mode and LP11,b mode. Analyze coupling characteristics and cross talk of the mode multi/demultiplexer using the beam propagation method. Numerical simulation shows that the operation bandwidth of the mode multi/demultiplexer can be 23nm if the normalized output power of LP11 mode in the few-mode core should be higher than -1dB, as well as the output powers of other modes all are lower -21dB above than that of LP11 mode in the few-mode core. The mode multi/demultiplexer can realize mode multiplexing and demultiplexing.