Abstract: Computer keyboards radiate electromagnetic（EM)disturbance noises unintentionally during working.Unauthorized person can recover keystrokes through receiving and analyzing these EM noises.From the experimental testing，we have found that the EM radiation of the keyboard mainly comes from the printed circuit board(PCB)and the transmitting cable.A common-mode voltage source is produced at the interface between the PCB and transmitting cable due to the unbalanced structure.And，this source can drive the PCB and the transmitting cable to radiate EM waves.Firstly，the principle of radiating EM waves from keyboard unintentionally is analyzed by using imbalance difference model.Then，based on this model the radiation from the PCB-cable interconnecting structure is calculated in theory and simulated through 3D full-wave EM simulation software.Finally，the keystrokes are recovered in the near-field through the experiment.The results demonstrate that the information of keystrokes are contained in the EM disturbance noises and can be eavesdropped in the near-field.