A fixed laser spot heat source, which can heat and cool amorphous alloys at a high rate, was used to re-melt the amorphous alloy Zr48Cu45Al17. The analysis of crystallization characteristics indicates that the heated-affected zone (HAZ) is liable to crystallize more easily than the molten zone (MZ), and at the boundary between the HAZ and the parent alloy there is a critical state between crystalline and amorphous state. Crystallization start diagram should be tangent with the thermal cycle of the critical location. Accordingly, the crystallization start diagram for amorphous alloy Zr48Cu45Al17 at high heating and cooling rate was determined with tangential points of the numerical simulated thermal cycles at the boundaries under different re-melting conditions