The asymmetrical fused-taper coupler is studied theoretically with weakly guiding and weakly coupling approximation in this paper, the analysis reveals that this kind of couplers have wavelength-flattened characteristics, and the degree of asymmetry can affect the splitting ratio directly. A wavelength-flattened coupler with different splitting ratio can be obtained by controlling the diameter ratio of two coupled arms in the process of fabrication. With 50% splitting ratio, the theoretical results agree with the experimental data.