The theoretical analysis of a flat-top AWG with multimode interference (MMI) power splitter at the input end is done. Several corrections to the former papers are made. For example, the field coupled into the arrayed waveguides is not treated with approximation, and domain of integration for coupling coefficient of an arbitrary waveguide in the array is chosen to be the waveguide spacing, which is coincident with the real optical propagation in AWG. Expressions for spatial distribution of field are obtained at the interfaces between waveguides, while an analytic expression for each channel response of AWG is derived. All of the expressions obtained are the functions of AWG configuration parameters. The output response of each channel is obtained by the analytic method much more quickly than by beam propagation method (BPM), and the period of optimization design of AWG is shortened greatly. Finally, a design example of flat-top AWG with good performance is presented, and its important specifications are calculated.