Micro/nano optical fibers have the optical property of large evanescent fields, which makes their coupling phenomenon more obvious compared with the normal optical fibers. The simulation of three parallel micro/nano optical fibers is performed by using the software of COMSOL Multiphysics which is on the basis of the finite element method. The results show that the coupling power distributions are approximation as the transmission distance changing when the distance between parallel micro/nano optical fibers is changed. The distributions are periodical and coupling periods increase when the distances increase. The polarization of incident light has an influence on coupling period and coupling efficiency. By utilizing the coupling characteristics among three parallel micro/nano optical fibers, a 3×1 micro/nano optical fiber coupler is designed and its coupling efficiency is approximately 93.3% when the appropriate coupling length and distances between the optical fibers are chosen.