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The nonlinear flutter of a narrow railway truss girder are studied through free vibration and forced vibration wind tunnel tests. Four functions are used to fit the structural amplitude-varying damping ratio under no wind condition, and the fourth-order polynomial has higher goodness-of-fit. The narrow truss girder shows nonlinear flutter of a velocity-restricted type without amplitude hysteresis. At large angles of attack (AoA), the vertical participation and the AoA have little effect on the onset wind speed and amplitude. While at medium AoAs, even a low vertical participation level will increase the onset wind speed and decrease the amplitude. At small AoAs, there is no vibration observed. The flutter derivative A 2 * is positive only in the limited reduced wind speed and amplitude range. The nonlinearity of self-excited lifting moment is higher only in the limited reduced wind speed and amplitude range, while that of self-excited lift is weaker and becomes stronger with the increasing amplitude and reduced wind speed. The negative aerodynamic damping is mainly provided by the flutter derivative A 2 * at small or large AoAs. Generally, the nonlinear flutter response is mainly controlled by the first-order component of the self-excited force, and the influence of high-order components can be ignored. • Relatively complete study of a narrow truss girder's nonlinear flutter characteristics. • The unique characteristics of the key flutter derivative A 2 * and self-excited lifting moment. • Analysis and verification of the influence of higher-order components of self-excited force on nonlinear flutter. [ABSTRACT FROM AUTHOR]