A pulsewidth tunable 10 GHz flat-top pulse train was generated based on the combined action of active mode locking and nonlinear-polarization-rotation pulse shaping. The laser cavity consisted of two parts: one was a commonly used active-mode-locking fiber laser; the other was for NPR based pulse shaping, which was composed of two PCs between which a high nonlinear fiber was inserted, and an in-line polarizer, was inserted into the cavity to control the pulse shape. The setup has previously been used for pulse compression, while the generation of FTP was proposed based on it. As a result, 10 GHz flat-top pulse with pulsewith tunable from 12~19 ps by tuning polarization controllers was generated, and flat-top pulse were obtained at different wavelength by tuning band pass filter. The output pulse shows good stability, with the measured side mode supress ratio of 65 dB and the timing jitter of 145 fs.