Abstract: A large number of crystals on the tunnel concrete substrate in the karst area will greatly increase the maintenance cost of the tunnel. Combined with theoretical analysis and test simulation methods, the process and principle of crystal formation and attachment on the tunnel concrete substrate were analyzed through observation of the changes in the microstructure of crystals on the surface and section of mortar blocks. The results showed that the growth process of calcium carbonate crystals on the concrete substrate could be split into 3 stages: initial stage, middle stage and later stage of calcium carbonate crystallization. At the initial stage, the concrete surface reduced the surface energy barrier at the time of nucleation, creating conducive conditions of heterogeneous nucleation for the nucleation of calcium carbonate crystals; at the middle stage, a great number of bonds were formed and a large amount of energy was released in uneven positions (holes, recesses, etc.) on the concrete surface, which was conducive to the stable growth of crystal nucleus, and crystals were more inclined to nucleate and adhere; after a transitional layer was formed on the concrete surface at the later stage, the calcium carbonate crystallization process was closer to homogeneous nucleation, and in this case, the solidification of crystal particles on the transitional layer accelerated and these particles were firmly bonded to the substrate..