Narrow pulse generation is one of the key technologies of ultra-wideband (UWB) radio wave communication. To simplify the photonic generation of UWB signal, a novel method for UWB microwave signal generation using a passively mode-locked fiber laser is proposed and demonstrated. Er3+-doped fiber as long as 3 m is used to get enough dispersion for achieving the wider pulse width, so that the frequency range of the pulse is between 3.1 and 10.6 GHz, as defined by FCC (US Federal Communications Commission). Based on the principle of harmonic mode-locking, the polarization is adjusted to get different periods of pulse so that the repetition rate is increased. In the experiments, the UWB signal is generated by beating the modes of passively mode-locked fiber ring laser at a wideband photo detector. A wideband numerical oscillograph is used to observe the wider pulse. The UWB photonic source with 8 different repetition rate outputs is gotten by adjusting the polarization controller.