Study on wind-induced vibration of long-span overhead transmission lines were among the most important subjects in Ultra-High-Voltage (UHV) power transmission projects. Comparing with Extreme-High-Voltage (EHV) and other voltage level lines, UHV long-span overhead transmission lines had suffered more special influence from wind gradient because of height requirements of waterway, structure of selected types of conductors and other factors. In this paper, functions of wind speed changed with both line coordinates and normal displacement was induced to establish three-freedom nonlinear dynamic equations, which described icing galloping of this type of lines more exactly. Considering actual structure and working condition of UHV long-span overhead transmission lines, this paper revealed the local bifurcation behavior and other dynamic characteristics of this model. The results showed that influence of wind gradient on the nonlinear dynamics of galloping of iced UHV long-span overhead transmission lines should not be neglected.