Reconnaissance and photography are important applications of unmanned aerial vehicles (UAVs). The terrain coverage task is performed by the fix-wing UAVs flying along the circuits in this article. Compared to the rotor UAV, the fix-wing UAV has a lower maneuverability, and its field of view is related to its flight direction. To avoid the frequent change of flight altitude, each UAV is assigned to fly at a flight level. In this cooperative terrain coverage task, the route of each UAV needs to be optimized, and the optimization objective is to minimize the sum of circuits’ length while satisfying the requirement on the coverage rate. To solve this problem, a random addition and deletion-based (A&D-based) algorithm is proposed, in which three key issues are addressed, i.e., initialization of feasible circuits, update of circuit, and calculation of the shortest route between two waypoints. Besides, a hierarchical online route planning algorithm is developed to cope with the change of no-fly zone and realize the local adjustment. The strategy of determining the start and end points of online route planning is presented, and the idea of transition point is introduced to obtain the shortest route while maintaining the coverage rate. Simulation results demonstrate that the A&D-based algorithm is effective in generating the circuit for each UAV, and the performance is better than the popular algorithms. The factors affecting the performance of algorithm are also analyzed. The hierarchical local adjustment strategy is valid in modifying the circuit in each flight level when facing different situations.