Abstract：Based on the features of the construction of two parallel tunnels excavated in horizontally interbedding rock mass，each rock stratum is regarded as the isotropic continuous medium，and the influence of rock stratum interface is considered，the numerical model is performed on the basis of the assumption. Adopting the FLAC，the stability of middle rock pillar is discussed mainly from the plastic zones and stress distribution of middle rock pillar. In addition，the influences of the different net distances between two tunnels and the various surrounding rock conditions on the stability of middle rock pillar are studied. Moreover，it is proposed that the improved method for calculating the safety factor of middle rock pillar is reasonable. The results show that the stability of the middle rock pillar will increase with the increasing net distances between two tunnels and in turns under soft rock，soft-hard interbedding rock and hard rock. It is available to judge the stability of the middle rock pillar in parallel tunnels under interbedding rocks on the basis of general analyses of the plastic zones，stress distribution and the safety factor of middle rock pillar. Furthermore，it is available to adopt the net distance of more than 1.0 time tunnel span under interbedding rocks，the middle rock pillar is stable and the especial reinforcement measures can not be adopted. The results are expected to be useful for the design of two parallel tunnels.
