To find out the effect mechanism of technological parameters on the performance of fiber coupler, the relationship between the technological parameters and the optical performance is investigated by using a six-axes optical fiber coupler machine as experimental setup. It is discovered that the technological parameters such as drawing speed and fused temperature have a great effect on optical performance of fibre coupler. The fused region and taper region of fiber coupler are tested with scanning electron microscope (SEM). It is discovered that there are crystalline grains in the fused region. And the crystalline grains decrease while drawing speed increase. In the taper region, there are micro cracks. And the cracks are more obvious while drawing speed increases. When the fused temperature is above some point (here is 1350), the micro cracks in the taper region disappear and the optical performance of fiber coupler is the highest at this time.