Abstract: The three-series solution is taken up to the nonlinear differential equation of the galloping movement caused by the cross wind force acting on a high-rise structure, thus giving a reduced 3D expression in terms of steady-state response and critical wind speed. Analyzes theoretically the vital factors which may lead to the occurrence of galloping oscillation as follows. The structure may form an airflow breakaway only if the slope of airflow lifting line is of a great negative value, i.e., the structure has a non-streamlined cross-section. The sufficient conditions to an instable structure are heavy mass, small stiffness and damping and tall with high aspect ratio. The equations derived shows that the nonlinear quadratic term of wind speed does not affect the value of critical wind speed but intensify the steady-state response and decrease the response frequency. It implies that the critical wind speed just depend on the first degree term of the speed, on which the initial few minutes to response to galloping oscillation almost depend. A numerical example indicates that the approach proposed is effective and easy to apply to engineering practice.