Fixed-wing unmanned aerial vehicle systems have good economic efficiency and can be used for point-to-point cargo transportation. This article investigates the design of the flight control law for unmanned aerial vehicles (UAVs) in the context of converting fixed-wing manned aircraft into fixed-wing UAVs. The design objectives of the control law is to achieve the tracking of the internal loop parameter commands and thereby realize the tracking of external loop parameters such as vertical speed and altitude. The architecture and switching logic of control law and inner-loop control law are designed, such that the expected values for the inner-loop attitude control are specified and tracked. By designing the outer-loop control law such as altitude control law, the external loop parameters are tracked. Finally, it is verified by the simulation results that the designed control law can achieve the aims of tracking the inner loop parameters and the outer loop parameters.