An accurate analytic expression of coupling coefficient between two parallel doubly cladding single mode fibers is given in this paper.The coupling coefficients as a function of normalized frequency parameter V are calculated for fibers with raised, matched, and depressed inner cladding indices. The coupling coefficients as a function of normalized distance D/a are also given with different V. The formula can be used to compute both the coupling coefficients for both x-polarized modes and the coupling of y-polarized modes further. It can also be used to analyze ilie coupling beliweeri two fibers with their large refractive-index difference, as well as polarisation obaraoterlstioe of optical fiber couplers.