Concerning the problem of trajectory tracking control for an Unmanned Aerial Vehicle (UAV) when the nominal values of autopilot parameters deviate from the actual values, a three-dimensional trajectory adaptive tracking control law was proposed. First, with no parameter deviation of autopilot parameters, a guidance law was derived and commands were obtained for the UAV's airspeed, track angle and flight-path angle. Global asymptotic stability of the closed-loop tracking system was proved by Lyapunov stability theory. Then regarding the deviation of autopilot parameters, a parameter adaption algorithm was designed to estimate the actual autopilot parameters online, and an adaptive tracking control law for UAV's three-dimensional trajectory was obtained. Simulation results show that the proposed adaptive tracking control law can achieve UAV's three-dimensional trajectory tracking effectively in spite of autopilot parameter deviation.