Abstract: Ramp Feed experiments performed on TRACY are simulated with the code Cri SAA which is used for the criticality excursion analysis. The transient characteristics of open criticality excursions are studied. The predictive results are shown. The statistics characteristics, including time and energy to first peak, power of first peak and the total energy, are obtained by simulations. The effects of the reactivity insertion rate and the uranium concentration on the criticality excursion are analyzed. Fuchs model are improved to be suitable for open criticality excursion in this paper. It is helpful to understand the open excursion.