Abstract: By using the thin film model,which is based on the brick-wall method,the entropy of the Dirac field in a general spherically symmetric and charged evaporating black hole is calculated.The conclusion that black hole entropy is proportional to its horizon area can still be applied by regulating the cutoff,which is time dependent.

Keywords: 
• entropy /
• evaporating black hole /
• thin film model /
• Dirac field /
• Dirac equation