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A novel method of optical Nyquist pulse generation based on recirculating frequency shift loop (RFSL) is proposed and experimentally demonstrated. In this article, a large number of flat phase-locked comb lines output from RFSL are used to synthesize Nyquist pulse, which has ultra-narrow and flexible tunable pulse width. It is the first time to theoretically analyze the condition of Nyquist pulse generation by RFSL in detail. In the experiment, the number of flat spectral lines is continuously tuned in the range of 10 to 40 corresponding to the pulse width of 8.9 ps to 2.4 ps. Meanwhile, Nyquist pulses with tunable repetition rate from 7.5 GHz to 15 GHz are obtained experimentally.