For dual-tapered fibers with non-absorbing refractive index, the mode-coupling equation for dual-tapered fiber coupler is derived, and the relationship among cross-coupling coefficients, self-coupling coefficients, interference-coupling coefficients and propagation constants is deduced by using the conjugated reciprocity theorem. Based on this relationship, the approximate solution to mode-coupling equation of dual-tapered fiber coupler with the identical propagation constant is solved. The analytical expression for the coupling optical power is obtained. Impacts of these coupling coefficients on the coupling optical power are discussed.