In this paper, the flutter instability of a conventional two-stage axially moving telescopic UAV wing is investigated. To this aim, and to be as close as possible to the reality, the effects of temporal variation of mass and length, due to the movement of stages and their overlapping, along with the effects of morphing speed are considered for the first time. The bending-torsional dynamics of the two-stage wing is modeled by modifying the Euler–Bernoulli beam theory to take into account the effects of morphing speed and variations of mass and length. Furthermore, the aerodynamic loads are simulated using Peters' unsteady aerodynamic model. The governing aeroelastic equations are discretized using a finite element approach, and a length-based stability analysis is proposed to investigate the aeroelasticity of the wing. The obtained results are compared with those available in the literature, and a good agreement is observed. It is found that the aeroelastic stability of a telescopic wing is more sensitive to the fixed part parameters than the moving part. Also, it is shown that the wing critical length is sensitive to the morphing speed. Therefore, by selecting the telescopic wing morphing parameters properly, the aeroelastic stability of the system can significantly be improved.