Abstract: The thickness of the overlying rock above the vault of a tunnel in the upper-soft lower-hard soil-rock composite stratum plays a key role in maintaining the self-stability of the surrounding rock. However, there has not been a unified understanding or standard on the safe thickness of the overlying rock of tunnels and its general laws.Based on this, this paper first investigates the safe thickness of the overlying rock of metro tunnels with different thicknesses of weak stratum Hs and excavation spans D through the finite element strength reduction method, obtains the mathematical fitting equation between Hrcv , the critical safe thickness of the overlying rock, and Hs, the thickness of weak stratum in metro tunnelling, and draws the "Hrcv - Hs - D" three-dimensional spatial distribution map of the critical safe thickness of the overlying rock, the thickness of weak stratum and the excavation span. Then,through the statistical analysis of the monitoring data during the construction process of Qingdao metro tunnels, it is verified that the self-stability of the tunnel surrounding rock and the degree of its influence on the surrounding environment are positive correlated with the safe thickness of the overlying rock.