Abstract: By building a horizontal condensing tube, the flow instability in condensing tube was simulated using RELAP5 program in this paper. The results show that the stability boundary of program calculation is in good agreement with that of Bhatt theory stability formula. Transient analysis result shows that vapor pressure and two-phase length in condensing flow do not match, which is the main reason of condensing flow instability and leads to outlet flow rate oscillation. Analysis of condensing tube parameters shows that smaller diameter and bigger flow area of condensing tube are of advantages to flow stability, but the influence of inlet flow rate is complicated.