Abstract: The secondary side of the nuclear power plant steam generator is working in a two-phase convective boiling process in which two-phase flow instability must be avoided in design. The time domain method was used to study the two-phase flow instability in a vertical up-flow tube. One-dimensional model was established and a numerical program was compiled. Two-phase flow density-wave instability was simulated, and transient parameter distribution in the tube during the dynamic process was presented. The program was also used to analyze the effects of mass velocity, system pressure, and inlet sub-cooling on flow instability. The result shows that the transient parameter distribution from the program agree well with the experimental results，and it can predict the instability boundary, which is more accurate than that of the usually used Khabenski nomographic method.