Abstract: Based on the structure characteristics of parallel channels, mathematical and physical models were established to simulate flow and heat transfer of the supercritical water in the parallel channels with semi-implicit method and staggered mesh method. Flow instability of supercritical water was obtained by using little perturbation method. The effects of mass flow, inlet temperature and inlet pressure on the flow instability boundary were also investigated. The stability of the parallel channels increases with inlet pressure and inlet flow, and decreases with the increase of inlet temperature.