Abstract: This paper analyzes the two-phase flow instabilities in parallel tubes using frequency domain control theory for steam generator in order to disclose the mechanism of flow instability. The mathematics expressions of heat transfer and flow of steam generator are proposed,and the eigenfunction of the close loop system is deduced using linearize perturbation theory and Laplace transformation. System stability is judged by Nyquist stability criterion of automatic control theory. On the above bases,the computer code is derived,and the stability analysis of steam generator is made by using this code. The results show that there is certain relations between the system stability and the channel number. The increasing of entrance resistance factor enhances the system stability; and the reducing of operation load impairs the system stability gradually.