We propose a novel ultra-wideband (UWB) triplet signal source based on the cross-gain modulation (XGM) in semiconductor optical amplifier (SOA). In the proposed scheme, only an optical source and two SOAs are needed, so the all-optical structure is compact. A triplet optical pulse with center frequency of 6.25 GHz and fractional bandwidth of 83% is obtained by the scheme. The extinction ratio can be improved by the counter-propagating scheme. The triplet pulse signal with only one wavelength can be easily controlled, and can aviod the dispersion effect. The output triplet pulse signal is insensitive to the light wavelength shifts, its available wavelength range is wide, the dynamic range of the input power is more than 6 dBm, and the bias current of the SOAs is exhibited.