Abstract: In order to exploit prior knowledge about the program code, a math model, Hidden Markov Model (HMM) , is applied to describe and resolve a problem of recovering the program code of a microcontroller.With the idea of reverse engineering and the method of Side-Channel Attacks (SCA) , and according to the property that different power consumption leakage characters for different instructions executed in chips, a practical experiment based on previous side channel templates built by making use of power consumption leakages for each instruction has shown that instruction sequences executed in a Data Encryption Standard (DES) microcontroller (AT89C52) cipher chip circuit can be reconstructed and has confirmed the effectiveness of method mentioned above.