The relation between coupling characteristic and numerical parameter of twin-core fiber (TCF) was analyzed with coupling mode theory. It’s proposed that the coupling characteristic of two different twin-core fibers can be the same so that the parameter of twin-core fibers can be optimized according to their application. Meanwhile,an improved fabrication method of twin-core fiber was presented. Twin-core fiber was fabricated by modified chemical vapor deposition (MCVD) and photonic crystal technology. Then,its numerical parameter was measured.