The application of unmanned aerial vehicles (UAVs) in urban areas enhances daily life convenience, but managing low-altitude urban airspace and mission planning remains challenging. This paper proposes a time-varying evaluation method for urban airspace management, combining airspace availability quantification and multi-risk assessment. First, a voxel-based measurement method was developed to quantify airspace availability by defining airspace coverage. Then, two risk assessment models were built: a static risk model and a dynamic risk model. The static risk model considers data transmission, collisions, indirect fatalities, and property damage; the dynamic risk model accurately quantifies direct fatalities and societal impact risks. Next, by integrating quadrant analysis with an improved Pareto ranking method, urban airspace below 40 meters was modeled and evaluated. Finally, a 3D urban airspace map with a resolution of 500 × 500 × 10 was generated, and airspace connectivity tests were conducted at 10:00 A.M. and 10:00 P.M. The results show that airspace connectivity is optimal between 28 meters and 40 meters at 10:00 P.M., improving with increased altitude.