Based on the gradient characteristic of taper of optical fiber and the change of degree of fusion between two fibers of fused fiber coupler in the drawing process, the changing model of waveguide structure of optical fiber coupler was constructed. The relationship between optical power and stretching length was obtained, and optical field distribution during drawing process was gotten by numerical simulation with beam propagation method. The simulation and experimental results show that hydrogen flow and drawing speed have an influence on degree of fusion. Large hydrogen flow and low speed make degree of fusion large. The coupling effect between fibers is related to the stretching length and the degree of fusion. Long stretching length and large degree of fusion make coupling effect strong. When the stretched length is short, the coupling of taper region can be ignored. On the one hand, it’s revealed fibers become thinner and coupling effect becomes stronger in taper region with the increasing of stretched length. On the other hand it is also showed that the energy leaks out of the fiber and it leads to excess loss.