Utilizing rectangular approximate in 3 dB fused-tapered optical fiber coupler, coupling ratio is analyzed theoretically. It is concluded that coupling length is the important element influenced coupling ratio. The relation between coupling length and coupling ratio is deduced. In the experiment, accuracy control of coupling ratio is gained by varying the coupling length. The experiment value is agreed with the theory value 0.04%/μm. Fused-tapered coupler which coupling ratio is accuracy 50∶50 is obtained.