Abstract: In order to study micronano fiber couplers, numerical simulation of the melting and drawing process of micronano fiber couplers was carried out by using beam propagation method. The curves of output light power with the change of tensile length and the distribution diagram of light field were obtained. The change of mode field and the characteristics of light field of three phases of the coupler were analyzed through software simulation. The results show that output light power of two fibers tends to be equal and no longer varies with the change of tensile length when a micronano fiber coupler is in the phase of losing the effective coupling. The characteristics of light field distribution of the fused taper coupler at various stages are different. The loss of effective coupling of taper-type micronano fiber coupler is directly related to fiber diameter of the melting zone. And the fiber diameter is related to the input light wavelength. The smaller the wavelength is, the smaller the fiber diameter that should be reached by the melt zone is. It is helpful for the study of the conditions for the loss of effective coupling of micronano fiber coupler.