Abstract: The electron back bombardment in the SW linacs with higher accelerating gradient is a undesirable phenomenon which will cause overheat of the electron gun cathode and destruction of the electron optics. The influences of the longitudinal motion on the electron back bombardment in SW linacs have been analysed by the same authors[1]. In this paper, the influ ences of the transverse motion on the electron back bamardment in SW linacs have been analysed by solving the beam envelope equations under backward motion. It is indicated that owing to the transverse motion under the backward motion, only part of the backward electrons can strike right on the cathode surface, the other part will strike on the inner walls of the cavity and beam hole. The definitions with their calculating methods of the back bombardment parameters such as back bombardment ratio, back bombardment current, back bombardment power are presented.