The flight dynamics of a morphing aircraft is obviously depended on the wing shape, because it results in different aerodynamic properties due to the mass reallocation and inertia change. This paper studies the flight dynamics of a changeable wingspan aircraft in low speed flight. Starting with the modeling of the morphing aircraft on the basis of Lagrange's equation, a set of longitudinal dynamic equations for a planar deployable wing is established. At the same time, the low speed aerodynamic parameters for fixed wingspan are calculated with the vortex lattice method. Afterward, the corresponding aerodynamic coefficients versus the wingspan changes are determined through the use of linear interpolation method. The effect of wingspan changes on the flight dynamics of the simplified morphing aircraft are analyzed for different flight status such as level flight, climbing, diving and turning. The results show that wingspan change would influence the flight status of morphing aircraft especially in the longitudinal flight so that the relevant control must be taken in order to keep the stability of the flight of a morphing aircraft.