Abstract: The electron back bombardment phenomenon, which results in anomalous increase of gun current emission, has been observed in a series of our side coupled standing wave accelerating tubes. In this paper, the electron back bombardment phenomenon is analysed using phase orbit equations deduced from the longitudinal motion equations for SW accelerating tubes. The motion of back-bombarding electrons in the first accelerating cavity are calculated under different conditions. The influences of various factors such as electric field distribution, amplitude of electric field and injection voltage are discussed, respectively.