In the identification of fire evidence, short circuit can be identified based on the metallurgical characteristics of the melted bead from the wire short-circuit. But because of the complexity in the real fire surroundings, short circuit melted bead is formed in many different ways. On the research, we analyze the microstructure characteristics of the short circuit melted bead in the condition of poor heat dissipation. By doing short circuit experiment in different cooling conditions, we can get the microstructure image of melted bead and compare them. Then analyze the difference and similarities and summary the variation law.