Urban low-altitude logistics delivery faces constraints such as dense buildings and complex airspace. Existing studies insufficiently characterize the geographical environment and treat task allocation and trajectory planning separately. Based on complex urban environments, this paper proposes a bi-level planning model for multi-UAV collaborative delivery. First, a 3D grid environment incorporating static obstacles and dynamic risk fields is constructed. Second, a bi-level coupled optimization model is established. The upper-level model aims at total cost and time satisfaction, employing an improved GA-SA algorithm for task allocation; the lower-level model introduces grid flight risk and integrates a 3D A* algorithm for collision-avoidance trajectory planning. Experimental results show that the model can effectively generate 3D cooperative obstacle-avoidance trajectories in complex urban scenarios, balancing timeliness and flight safety.