View-dependent level-of-detail (LOD) and visibility culling are two powerful tools for accelerating the rendering of very large models in a real-time visualization, especially in walkthrough of a large-scale terrain environment. In this paper, we propose a visibility-driven Continuous LOD (CLOD) framework for terrain, which takes advantage of both techniques. The visibility determination is based on the well-known occlusion horizon algorithm. By making use of the features of the terrain extracted in pre-processing stage, a new cascading occlusion culling (COC) algorithm is proposed to cull those regions classified as invisible to current viewpoint in real time. The time consumption and storage overheads that we spend on visibility preprocessing are quite small. Visibility-driven CLOD enhances culling efficiency and improves the frame rates significantly for walkthrough of a terrain environment.