Choose the appropriate continuous function to describe the light field distribution of fusion biconical optical microfiber coupler which is made by a fusion tapering system. The weighted superposition of normalized triangular and rectangular distributions combine the weighted superposition of Gaussian and triangular distributions are used to achieve continuous changes in the mode field along the coupler region. The local mode coupling theory is used to derive the calculation formulas for the coupling coefficients of the optical microfiber coupler waist region and the tapered region near the waist region, and get the change curve of optical microfiber coupler output optical power with stretched length. Calculation results show that as the stretch length increases, the degree of light energy coupling back and forth alternately in both arms becomes smaller and appears envelope-like, until the coupling function of the waist region disappears. Among monitoring real-time data of the optical microfiber couplers′ output optical power, the effects of flame scan width and hydrogen flow on the length of two arms loss coupling effect are obtained. The flame scan width is wider, the uniform waist region is wider, the critical value of the stretch length is larger. The greater the hydrogen flow, the greater the melting degree, the smaller the critical value of the stretch length. The experimental results show that when the diameter of the fiber coupler reaches 1.6 μm, the coupling function disappears, the optical power of the two output ports is the same and constant, and the optical microfiber coupler has stable optical transmission characteristics.