The analysis of optical mode field in the coupler was carried out,is in the condition of weakly guiding,weakly coupling,an approximate shape of fused tapered coupler and the appropriate continuous functions,description. Triangle function and Gauss function are mixed with variational coefficients to describe it. Base on local mode theory and variational technique,propagation constant and coupling coefficient are analyzed in each part of fused tapered coupler. The relations between coupling coefficient,wavelength and geometric coefficients are revealed. Numerical simulation indicates that power coupling in the taper region must be taken into account. The coupling coefficients are affected by core and cladding,besides,the effects take place in the taper region and the crossing region in the opposite way. Thereby,a wavelength-flatten symmetry fiber coupler can be obtained through controlling the shape coefficients suitably.