Coupling La –doped BaF2 crystal to phototube is an effective way to prevent signal pileup in ultra –fast radiation detector under strong g field. The ratios of slow components to fast ones of domestic BaF2 crystal and La –doped BaF2 crystal have been investigated through Synchrotron Radiation Decay Time Spectroscopy and X ray Excited Emission Spectroscopy. The mechanism of the slow component suppression by doping with La and the anti-irradiation property of BaF2（La）are studied. The slow component suppression of ultra –fast radiation detector, which was composed of BaF2（La）and GD40Z phototube is measured through Synchrotron Radiation.