Because of the small space range，slow target speed， and mixed environmental elements of low-altitude flight， the traditional latitude and longitude characterization cannot meet the requirements of low-altitude fine management in the Smartlink environment. Therefore the digital low-altitude airspace raster characterization metrics and optimal calibration problems are studied. Firstly， the quantitative characterization rules of multi-dimensional low-altitude airspace structural elements are constructed from the perspective of “point-line-plane”， the quantitative characterization method of multi-level raster in low-altitude airspace is proposed. Then， by determining the “point-line-plane” positional relationships of different airspace rasters， we propose a topological relationship metric of low-altitude airspace based on the raster intersection matrix. Finally， considering the optimization objectives of low-altitude unmanned aerial vehicle（UAV） collision index and low-altitude raster utilization index， as well as the node-raster matching constraints， spatial position constraints， and safety constraints of UAVs and UAVs/obstacles， we establish a multi-dimensional performance oriented optimal calibration model for the raster granularity of the low airspace， and evaluate the effectiveness and efficiency of the proposed method for the typical mission scenarios of the low airspace. The validity and optimization effect of the proposed method are verified and analyzed for typical urban low altitude flight scenarios. The experimental results show that the proposed method can optimally configure the digital low altitude raster granularity for any low-altitude airspace and UAV mission with acceptable UAV collision index and raster utilization index， so as to realize the safety and high efficiency of low altitude flight activities. The research results have certain theoretical value and application significance to support the fine management of digital low altitude airspace and the fusion operation of heterogeneous aircraft.