Abstract: With its disadvantages such as inflexibility and poor security,the traditional hardware structure of S-box can only be suitable for a cryptography algorithm.By analyzing the characteristics of most of the block ciphers,a new S-box circuit structure is designed,whose size can be reconfigured into 4×4 or 6×4 with same hardwares.To improve the adaptability of S-box to different cryptography algorithms,the hardware multiplexing technology is adopted,and the proposed reconfigurable S-box has a faster reconfiguration speed and better expansibility.To reinforce the security of S-box,genetic algorithm is adopted to optimize the reconfigurable S-box circuit.For the construction of the fitness function of genetic algorithm, nonlinearity and difference uniformity are mainly considered,which affect the security of S-box.Taking the 4×4 S-box for example,the experiment results verify the good flexibility and security of the reconfigurable S-box,and also check the correctness of the reconfigurable S-box circuit structure and the optimum design method.