Fused-taper fiber coupler is usually fabricated by using a microburner. The coupler’s excess loss and performance repeatability are unstable. To simulate the fabrication process and to find solution to the performance questions, A viscoelastic rheological model of fused biconical taper process is presented by using generalized Maxwell model to simulate the viscoelastic characteristic of fused fiberglass, and the rheological numerical analysis under the non-isothermal condition is carried out with ANSYS software. The effect of technical parameters such as drawing speed and fused temperature on the fabrication process is studied. Drawing speed is in direct proportion to the maximum stress, and fused temperature affect extremely the fabrication process. The fluctuation of fused temperature by 5 can lead to the change of maximum stress by 30%. It is found by experiments that the flame temperature fluctuates in the range of 5-30, while the drawing speed is rather stable. It can be reached a conclusion that it is the fluctuation of fused temperature that induces the poor performance repeatability and unstable excess loss.