According to the microstructure of amorphous crystal, the percolation theory, which is a theoretical approach to dealing with the inhomogeneous physical systems or random fractals, is used to describe the plastic flows of amorphous alloys under shear yielding. In order to understand in depth the critical problems about the shear band initiations in amorphous alloys, a percolation model for shear transformations of these alloys is established by combining with the existing free volume model and shear transformation zone model. Taking the binary amorphous alloy Cu25Zr75 for example, the percolation threshold for the shearing instability of the atomic clusters prone to producing plastic flows in the shear transformation zone is calculated when a shear band comes into being. In addition, the size of the above-mentioned cluster is also roughly estimated. The calculated results show that the percolation threshold of the shearing instability is similar to the critical reduced free volume value (xC) of~2.4% for the onset of yielding in amorphous alloy although this threshold is closely related to the dispersity of free volume. The present study may provide a new idea and method of studying the ductile-brittle transition in amorphous alloy.