Effect of the mutual coupling among waveguides on propagating and scanning characteristics of waveguide array electro-optic scanner (WAEOS) is studied by use of the finite-difference beam-propagation method (FD-BPM). The results indicate that the mutual coupling is usually of great influence on these characteristics but it is of little influence on them only under some certain conditions. Therefore, in order to design structure of WAEOS appropriately, it is necessary to consider the effect of the mutual coupling on propagating and scanning characteristics fully. Some parameters, which are used to describe performance of WAEOS, are put forward and then the optimal structure of WAEOS is discussed with considering the mutual coupling fully.