Abstract: The control equations of the calculation model on flow instability for a supercritical water parallel channel system were established.Semi-implicit finite-difference method with staggered mesh was used to discretize the control equations solved by the main element Gauss-Jordan elimination method.The accuracy of this model was verified by the experiment data.Uniform power distribution was selected as the reference axial power distribution,and cosine,bottom-peaked and top-peaked power distributions were used to simulate the axial non-uniform power distribution.The marginal stability boundaries（MSBs） for those four different axial power distributions were obtained by the transient calculation.The results show that compared with the uniform power distribution,the bottom-peaked power distribution will destabilize the system stability,while the top-peaked power distribution can enhance the system stability in the whole pseudo-subcooling number region.In high pseudo-subcooling number region,cosine power distribution will decrease the system stability,while the system stability will be enhanced in low pseudo-subcooling number region.In addition,a concept of pseudo-single-phase pressure drop ratio was proposed to analyze the influence mechanism for axial power distributions on the flow instability of the supercritical water in parallel channels.