A kind of three modes tapered fiber mode multiplexer/demultiplexer is developed based on mode coupling theory for realizing modes multiplexing and demultiplexing between a few-mode fiber (FMFs) and three single-mode fibers (SMF). It′s significant to improve the transmission capacity of optical transmission system. When the fundamental mode lights are input into three SMFs with the diameters of 10 \mm, 8.4 \mm and 7.2 \mm, the lights will couple in the tapered area and convert into higher order modes. The similarity between optical field at output port and standard mode field is further calculated by overlap integral. As a consequence, the fundamental modes can be converted into LP01 mode and two degenerate LP11 modes at the output port, i.e. realization of modes multiplexing. When the LP01 mode and two LP11 modes are separately input into the FMF, the output light power of corresponding port is the maximum, i.e. realization of modes demultiplexing. Finally, the multiplexer's tolerance is calculated with different parameters. Calculation results show that 3.88 cm and 3.8931 cm are the tapered structure′s length of the multiplexer for best effect of mode converting and demultiplexing.