The invention discloses a waterproof and drainage system capable of preventing completely weathered granite slope water damage and a construction method of the system. According to the system, imitation natural step type cut slope patterns serve as the basis, and the system is formed by an anti-seepage platform (12), a platform side ditch (9), a seepage stopping blind ditch (11) and slope three-dimensional planted grasses (10). By means of the system, most souring action and infiltration capacity caused by rainfall within the cut slope range can be eliminated, the system has the advantages of being low in cost, simple to construct, high in anti-draining efficiency, good in environmental protection property and the like and particularly applicable to prevention and treatment of thick-layer weathered granite cut slope deep water damage and shallow landslide damage caused by cut slope deep water damage.