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The multiple phase shifts technique is introduced into digital concatenated grating to design fiber grating comb filters. Detailed analysis is given for the first time. We find that by increasing the channel multiplication factor and concatenated grating segment number, reflection spectrum can be broadened and channel density can be increased without increasing the sampling period. Comb filters covering the C -band with different channel spacing (200, 100, and 50 GHz) are designed. An ultrahigh channel-count comb filter covering the S-, C-, and L-bands with 100 GHz channel spacing is also demonstrated. Simulation results indicate this technique is a promising method for designing ultrahigh channel-count comb filters.