We present an efficient approach to design a high-channel-count multichannel fiber Bragg grating by assigning optimal sets of delay coefficients and constant phases to the corresponding channel responses. Based on approximate Fourier transform, the delay coefficients are chosen to separate all the single-channel gratings into several groups spatially in the grating structure, and the constant phases in each group are optimized to minimize the maximum index modulation to be approximately the square root of the maximum of the number of the channels in all groups times larger than that of the one-channel grating. Design examples demonstrate that the proposed method has advantages of low index modulation, low algorithmic complexity, and suitability for multichannel fiber Bragg grating designs with either identical or nonidentical spectral responses.