Abstract: Aiming at the simplified model of supercritical water cooled reactor thermal-hydraulic system, small perturbation linearization and Laplace transform method were adopted to linearize the nonlinear thermal-hydraulic system conservation equations. Then the closed-loop system transfer function was deduced. Matlab code was used to analyze and simulate the closed-loop system and obtain the stability boundary map of the closed-loop system, and the effects of reactor core inlet flow velocity, heating length, gravity-acceleration and inlet throttling coefficient on the system stability boundary were analyzed finally. The results show that if the reactor core inlet flow rate, the heating section length, and the gravity acceleration increase, the stability of the system will be better, and however the inlet throttling coefficient rarely affects the stability boundary.