Two-sided lapping is an essential process in the fabrication of sapphire substrates， and it significantly affects the amount of material removed during subsequent polishing. Therefore， studying the characteristics and measuring the depths of surface cracks on lapped substrates are important processes. In this study， we investigate the surface crack characteristics and crack depths of two-sided lapped sapphire substrates using section apparent micrometry， focused ion beam side observations， a differential etching rate method， a magnetorheological polishing method， and a layer-by-layer polishing method. Consequently， we observe subsurface cracks on the sapphire substrate after grinding using the cross-sectional microscopic observation and focused ion beam side observation methods. These cracks mainly include oblique lines， horizontal lines， hooks， and dendritic patterns. The differential etching rate method reveals that the thickness of the crack dense layer on the grinding surface of the sapphire substrate measures 9-10 μm. Using the magnetorheological polishing method， we measure the depth of local subsurface cracks on the grinding substrate to be 25-30 μm. Furthermore， employing the layer-by-layer polishing method， we determine that the overall subsurface crack depth of the ground substrate is approximately 30-35 μm. Additionally， based on the crack characteristics and depths detected using different methods， we construct a surface crack model for the two-sided lapping of sapphire substrates. This model serves as a foundation for formulating and optimizing subsequent polishing processes.