Abstract: Electromagnetic side channel attack is an effective method in side channel attack. In order to overcome the limitations of traditional electromagnetic side channel attacks that must be acquired at close distance, a far field attack method based on correlation analysis is proposed for cryptographic devices without electromagnetic protection. The microcontroller running the advanced encryption standard(AES) and the antenna detecting the signal in the far field were used in our experiment. Before perform the side channel attack, we take the average value and filter the collected signal to reduce the influence of noise. Successfully we crack all 16-byte keys at 10 m. At the same time, the frequency and sample size of the far field electromagnetic side channel attack are deeply studied. In fact, the electromagnetic side channel with key information mainly concentrates in a frequency range, and the the key information is more obvious with the increase of sample size. Based on this, an improved electromagnetic side channel attack method is proposed combining with the mechanism of key information leakage generated by cryptographic chips.