Abstract: The stability and chaotic motion of a pipe conveying fluid with both ends fixed, excited by the harmonic motion of its supporting base in a direction normal to the pipe span, were investigated theoretically. The nonlinear differential equation of the system motion was derived by considering the additional axial force due to lateral motion of the pipe. Melnikov method was used to predict the initiation of chaotic motion. The critical value was found to be smaller than that determined by numerical simulation. The motions of both single-mode and two-mode systems were analyzed by numerical simulations The high-order mode of the system can quantitatively complement the displacement of the single-mode system.