In recent years, some lightweight cipher algorithms for IoT devices have been proposed. These algorithms can be applied to a variety of IoT devices without occupying too much memory and power consumption, and compared to traditional ciphers, such as AES, these algorithms have higher efficiency. The Speck algorithm is a lightweight block cipher, proposed by the NSA in June 2013, and it is specifically optimized for IoT microcontrollers. A few previous studies have shown that the Speck algorithm can be attacked through power analysis. Based on Arduino Uno as the implementation platform of the Speck 128/128 algorithm, in this paper, we propose a method to attack the Speck 128/128 algorithm. Our experiment results have shown that when the attack point in the last round of Speck 128/128 XOR operation, the round key can be successfully recovered by the Correlation Power Analysis attack.