Abstract: Based on the synthesis of all sorts of factors influencing the dynamic characteristics of steam generator（SG） U-tubes,the natural frequencies,corresponding mode shapes of the U-tubes are analyzed un-der practical load cases and environment by theoretical analysis and numerical simulation.The formulae of calculating the natural frequencies of U-tube considering the effects of internal and external pressures,as well as non-uniform distributed mass density in U-tubes are deduced.Study results reveal that the boundary constraints,pressures in primary and secondary sides,supporting width,support plate number and elbow tube factor bring a strong effect on the dynamic characteristics of U-tubes,and the non-uniformities of pressures and mass along the whole tube,tube deadweight and the heat expansion have little.