Taking the coaxial twin-rotor UAV as the research object, an adaptive Super-Twisting UAV attitude controller is designed to tackle the strong nonlinearity of the coaxial twin-rotor UAV system and the uncertainties generated during flight. Firstly, the mathematical model of the coaxial UAV is established by analyzing its flight states. Secondly, with the errors of attitude angles and attitude angular rates as state variables, a corresponding sliding mode surface is designed, and an adaptive Super-Twisting attitude controller is adopted. Then, stability analysis of the UAV system is performed. Finally, algorithm simulation tests are conducted. The experimental results indicate that, compared with the traditional sliding mode control algorithm, the attitude control algorithm proposed in this paper can effectively enhance attitude stability and response rapidity.