Abstract: In the high power klystrons, the AC space-charge effects from the bunching of electrons in gap can not be ignored due to the higher beam current and longer gap distance. The traditional coupling coefficient calculation model according to the simplified kinematic theory does not take the effects in account, so its result is higher than the actual value. This paper develops a new coupling coefficient model based on the Webster debunching theory considering the AC space-charge effects with arbitrary gap field distributing. In addition, a simulation study of the coupling coefficient is given using the particle-in-cell code. The simulation results show agreement well with the calculating results.