Considering the effect of the internal and external fluid excitation, the flow-induced vibration stability of fluid-conveying pipes were investigated. The role of the external fluid was simplified as vortex-induced lift, and the nonlinear differential equations were derived based on Kane’s equation. The equations of motion are linearized in the neighborhood of the equilibrium position to carry on the multimode linear stability analysis. The effect of internal and external fluid velocities, fluid-added damping coefficient, and span length on the vibration stability is discussed. The results obtained show that non-linear flow-induced vibration model is a true reflection of the fluid-induced vibration stability of fluid-conveying pipes. Under the influence of internal fluid and span length, the coupled-mode flutter takes place; The external fluid-added damping coefficient changes has significant impact on the dynamic characteristics of the pipe；under the combined effects of fluid inside and outside the pipe, vibration characteristics significantly different from the role of each factor alone.