The invention discloses an output feedback type sigma-delta modulator for maximizing an output cycle length. The modulator adopts three output feedback modules (OFM) same in structure and a noise cancellation network N; for the modulator with the resolution of n bit, the OFMs of the modulator feed back an output to the output cycle length 2<3n> with a maximum input, so that the dispersion of a fractional-N frequency synthesizer can be effectively reduced; and the shaping of 60dB in each tenth harmonic generation is generated for noises by adopting a three-level cascade structure, so that in-band quantized noises are reduced.